DO THE TON

Blood Sweat Tears and Grease => Engines => Topic started by: Midcoastcustoms on Jun 27, 2010, 16:03:58

Title: Motorcycle Carburetor Theory 101
Post by: Midcoastcustoms on Jun 27, 2010, 16:03:58
*NEW* LINK TO PDF FILE *NEW*
http://www.motorcyclecarbs.com/carbs101.pdf (http://www.motorcyclecarbs.com/carbs101.pdf)
Motorcycle Carburetor Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image001.gif)
FIG 1

Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image002.gif)
FIG 2

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image003.gif)
FIG 3

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image004.gif)
FIG 4

The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image005.gif)
FIG 5

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.

 
(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image006.gif)
FIG 6

Carburetor Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image007.gif)
FIG 7

While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color. If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.

 

Altitude, Humidity, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performance of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32š Fahrenheit may run poorly when the temperature reaches 90š Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found. Using fig 8 as an example, the air temperature is 95š Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image008.gif)
FIG 8

Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.

Needle Jet/Jet Needle/Air Screw Correction Chart

 

Correction factor     1.04 or above        1.04-1.00        1.00-0.96        0.96-0.92         0.92 or below

Needle jet               2 sizes larger     1 size larger      Same size       1 size smaller    2 sizes smaller

Jet needle setting    Lower clip position    Same              Same               Same          Raise clip one position

Air screw opening   One turn in            1/2 turn in          Same          1/2 turn out      One turn out

All info is from this parts site  http://www.siriusconinc.com/index.php (http://www.siriusconinc.com/index.php)

 
Title: Re: Motorcycle Carburetor Theory 101
Post by: boingk on Jun 28, 2010, 18:31:59
Nice work mate. I know all this from dicking around in the garage and whatnot, but it'll be a kicker of a resource for others.

Good work on taking the time to write it up and put some decent diagrams in there. Very clear and concise.

 - boingk
Title: Re: Motorcycle Carburetor Theory 101
Post by: dcmspikes on Jun 28, 2010, 21:42:31
thank you
Title: Re: Motorcycle Carburetor Theory 101
Post by: Midcoastcustoms on Jun 28, 2010, 22:26:56
Nice work mate. I know all this from dicking around in the garage and whatnot, but it'll be a kicker of a resource for others.

Good work on taking the time to write it up and put some decent diagrams in there. Very clear and concise.

 - boingk
That's how I learned too, by doing, I grew up in the garage...all the info was from the link at the bottom of the page.  I came across it while searching for parts and thought this would be of help to many. 
Title: Re: Motorcycle Carburetor Theory 101
Post by: locO leoN on Jul 04, 2010, 13:13:16
Wow!!..
What a fantastic thread!.
Thanks for taking the time to write it up!!.

I'm gonna sticky this thread!!.
Title: Re: Motorcycle Carburetor Theory 101
Post by: Midcoastcustoms on Jul 05, 2010, 22:56:09
Wow!!..
What a fantastic thread!.
Thanks for taking the time to write it up!!.

I'm gonna sticky this thread!!.
Once again i didn't write this just re-arranged the info and posted it.  I found this while looking for parts for my cb450.  The info came from this site http://www.siriusconinc.com/faq.php#name_7 (http://www.siriusconinc.com/faq.php#name_7)  This was probably one of the best carb theory explanations i have seen.  Hope this is of great help to many!
Title: Re: Motorcycle Carburetor Theory 101
Post by: Cat13 on Jul 07, 2010, 18:59:22
Thank you for posting this. Now I've gone from knowing an itsy bit about carbs to knowing a whole lot more.
Title: Re: Motorcycle Carburetor Theory 101
Post by: MrNooberton on Jul 11, 2010, 19:13:14
Thank you very much for posting this!
Title: Re: Motorcycle Carburetor Theory 101
Post by: chris73cb175 on Aug 19, 2010, 15:07:41
this should stay up forever hahaha. thanks for the info i'm sure it has helped a lot of us trying to tune our bikes
Title: Re: Motorcycle Carburetor Theory 101
Post by: Will McD on Sep 03, 2010, 20:33:32
Best jetting post I have read.

However, my bike is giving me problems, and I can't figure out where my problem fits in all of this.

When I bought the bike it had sponge-like unifliter pods on each carb of my 82 CB650.
I installed K&N pods instead.

The bike runs pretty good, i have never ridden a bike of this size so I can't be sure. It idles fine, and seems to have good power. I have gotten it up to the ton no problem with more rpms to spare.

HOWEVER, today I finally put insurance on it, and plated it, and got ready to go on a road trip this weekend and I took it out for a ride. It was moderately windy (more wind than this bike has seen in the short time i've had it). It ran great going with the wind. No problem. When I turned around to go against the wind the bike ran like crap. Whenever a gust of wind hit me the bike bogged down so bad I almost had to pull over. I tried running with the choke half way and it seemed to fix the issue and it ran pretty decent.

So what is my issue? I suppose the bike is running lean since the choke helped.
Why does the bike run fine with lots of power as long as there is no wind?
I want to fix this problem fast for the weekend. I might try to tape up the air filter pods to limit some air and see if that helps.
Which jets would I be looking at changing in this case?

I think I'll post this under it's own subject too.

Thanks!
Title: Re: Motorcycle Carburetor Theory 101
Post by: callmenames on Sep 04, 2010, 03:28:53
Thanks for the PDF, I want to go Pod Filters and had no idea what size to go for my jets, this should make things a lot easier to figure out.
Title: Re: Motorcycle Carburetor Theory 101
Post by: Kyzer on Nov 11, 2010, 08:32:28
Good Morning.  Your explanation was very deep.  Thank you for taking your time to help us understand.

My question stems from the "slider valve."  I have a '67 CB160 that I am fixing up.  Mine was sticking, causing the throttle to not snap back on the handle.  When I took it out to clean it, and put it back in, it now doesn't seem to fit or slide back into the chamber.  I am afraid if I push it in too hard it will get jammed.  Am I doing something wrong? ???

Thank you.


*NEW* LINK TO PDF FILE *NEW*
http://www.motorcyclecarbs.com/carbs101.pdf (http://www.motorcyclecarbs.com/carbs101.pdf)
Motorcycle Carburetor Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image001.gif)
FIG 1

Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image002.gif)
FIG 2

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image003.gif)
FIG 3

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image004.gif)
FIG 4

The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image005.gif)
FIG 5

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.

 
(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image006.gif)
FIG 6

Carburetor Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image007.gif)
FIG 7

While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color. If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.

 

Altitude, Humidity, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performance of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32š Fahrenheit may run poorly when the temperature reaches 90š Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found. Using fig 8 as an example, the air temperature is 95š Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image008.gif)
FIG 8

Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.

Needle Jet/Jet Needle/Air Screw Correction Chart

 

Correction factor     1.04 or above        1.04-1.00        1.00-0.96        0.96-0.92         0.92 or below

Needle jet               2 sizes larger     1 size larger      Same size       1 size smaller    2 sizes smaller

Jet needle setting    Lower clip position    Same              Same               Same          Raise clip one position

Air screw opening   One turn in            1/2 turn in          Same          1/2 turn out      One turn out

All info is from this parts site  http://www.siriusconinc.com/index.php (http://www.siriusconinc.com/index.php)

 
Title: Re: Motorcycle Carburetor Theory 101
Post by: Midcoastcustoms on Nov 12, 2010, 02:25:12
Not sure but it may be off 180 degrees.  Repost what u wrote under a new topic in 1-800- cafe help with pics and i'm sure many people will be able to help including myself.  No one will see the post here.
Title: Re: Motorcycle Carburetor Theory 101
Post by: dirtyc88 on Aug 15, 2011, 01:33:41
hey man thanks for posting what you found im really stoked hope what i learned here will help me get my bike running right
Title: Re: Motorcycle Carburetor Theory 101
Post by: bonez1122 on Aug 29, 2011, 18:01:20
looking to put pod filters on my carbs on cb450 any1 know what size jets i need to put in my carbs to keep running?? and any1 have any 2 sell?
Title: Re: Motorcycle Carburetor Theory 101
Post by: donald branscom on Oct 27, 2011, 03:54:41
Do those carbs have an accelerator pump?
If they do and you look in the intake (engine off) and you turn the throttle you
should see a squirt of gas come out of the brass tube sticking up.


Can you please post a photo of the carbs.
Then I can tell you where the problem is.
Title: Re: Motorcycle Carburetor Theory 101
Post by: donald branscom on Oct 27, 2011, 03:57:33
looking to put pod filters on my carbs on cb450 any1 know what size jets i need to put in my carbs to keep running?? and any1 have any 2 sell?


At least one step up from stock. Also the needles may need to me raised.
Need more info about the type of exhaust system.


The workshop manual tells you the stock jet size for California,49 state and high elevation states like Colorado.
You HAVE TO HAVE a  workshop manual for any motorcycle you are working on.
Title: Re: Motorcycle Carburetor Theory 101
Post by: donald branscom on Oct 27, 2011, 04:01:52
Good Morning.  Your explanation was very deep.  Thank you for taking your time to help us understand.

My question stems from the "slider valve."  I have a '67 CB160 that I am fixing up.  Mine was sticking, causing the throttle to not snap back on the handle.  When I took it out to clean it, and put it back in, it now doesn't seem to fit or slide back into the chamber.  I am afraid if I push it in too hard it will get jammed.  Am I doing something wrong? ???

Thank you.


The slides have a front and a back.
There is a notch that has to line up.
The cut away in the slide has to be on the air cleaner side of the carb.
Since it is a CB160 .Did you get the left and right carbs and slides mixed up?
Check that out.
Title: Re: Motorcycle Carburetor Theory 101
Post by: donald branscom on Oct 27, 2011, 04:03:14
Best jetting post I have read.

However, my bike is giving me problems, and I can't figure out where my problem fits in all of this.

When I bought the bike it had sponge-like unifliter pods on each carb of my 82 CB650.
I installed K&N pods instead.

The bike runs pretty good, i have never ridden a bike of this size so I can't be sure. It idles fine, and seems to have good power. I have gotten it up to the ton no problem with more rpms to spare.

HOWEVER, today I finally put insurance on it, and plated it, and got ready to go on a road trip this weekend and I took it out for a ride. It was moderately windy (more wind than this bike has seen in the short time i've had it). It ran great going with the wind. No problem. When I turned around to go against the wind the bike ran like crap. Whenever a gust of wind hit me the bike bogged down so bad I almost had to pull over. I tried running with the choke half way and it seemed to fix the issue and it ran pretty decent.

So what is my issue? I suppose the bike is running lean since the choke helped.
Why does the bike run fine with lots of power as long as there is no wind?
I want to fix this problem fast for the weekend. I might try to tape up the air filter pods to limit some air and see if that helps.
Which jets would I be looking at changing in this case?

I think I'll post this under it's own subject too.

Thanks!





Do those carbs have an accelerator pump?
If they do and you look in the intake (engine off) and you turn the throttle you
should see a squirt of gas come out of the brass tube sticking up.


Can you please post a photo of the carbs.
Then I can tell you where the problem is.
Title: Re: Motorcycle Carburetor Theory 101
Post by: teazer on Nov 11, 2011, 13:45:39
CB160 carbs are very simple.  Take teh slide out again and see why it's sticking.  It may be rotated out of position or the needle may not be going into the needle jet correctly.  It should be fairly obvious if you take your time to look.
Title: Re: Motorcycle Carburetor Theory 101
Post by: TonUpSoldier on Nov 29, 2011, 20:10:39
CB160 carbs are very simple.  Take teh slide out again and see why it's sticking.  It may be rotated out of position or the needle may not be going into the needle jet correctly.  It should be fairly obvious if you take your time to look.


Also, if you've used a dip type substance to clean the carb (which you should. ALWAYS assume the carb is dirty if youve never cleaned/rebuilt it. And be sure to replace any and all Orings inside...they dryrot, deteriorate from ethanol, etc..they're sealing devices) and didnt rinse it thoroughly, the residue left from the dip will cause the slide to stick. If you left it in the dip to long, depending on the chemical make-up of the dip (the crap they sell now is WEAK SAUCE...stupid EPA) can cause a oxidizing type reaction in the slide bore (cheap alu/zinc alloy...god help you if it ever sits in water for long periods of time...) which will cause stiction. Clean the bore out well with some 000 or higher steel wool...make sure you blow it out well with compressed air or the like dont leave that wool remnants in the bore. Should rectify your problem...
Title: Re: Motorcycle Carburetor Theory 101
Post by: VJ400 on Jan 04, 2012, 18:54:27
THis answers sooooo many questions for me. HAHAHA!!!
Title: Re: Motorcycle Carburetor Theory 101
Post by: bajabill on May 02, 2012, 20:04:05
looking for carb diaphrams go to jbnindustries  they are just 19.00 each shipping is 4.00 hope this helps all you at dtt
Title: Re: Motorcycle Carburetor Theory 101
Post by: bajabill on May 02, 2012, 20:07:48
sorry they are 19.50
Title: Re: Motorcycle Carburetor Theory 101
Post by: sham on Jun 17, 2012, 10:22:43
I also came across this article, which deals more with carb tuning, but it seems quiet good to me (I haven't done any of this).


http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
Title: Re: Motorcycle Carburetor Theory 101
Post by: Sonreir on Jan 12, 2013, 18:59:43
Topic moved to the Engines section.
Title: Re: Motorcycle Carburetor Theory 101
Post by: sxecafe on Mar 06, 2013, 13:02:21
Ok guys... I'm stumped on something. I've been blasting around on the scoot ('74 CB550) and trying to tune things up. I am running pods, dual 2-1s that have been chopped down. I have stock jet sizes and around 1/2 throttle I'm loading up heavy and can barely pull out of it to get any power back. To me... with those modifications, I would feel like I would need to jet up but I pull the plugs and they look like this:

1 & 4, respectively.

(http://i304.photobucket.com/albums/nn180/DIRescue/432A080B-E01B-41A5-8B67-621CE1D8AED7-15227-00000870350E018B_zpsefd224ee.jpg)

They're black as Wesley Snipes! How could that be? Float levels maybe?
Title: Re: Motorcycle Carburetor Theory 101
Post by: teazer on Mar 06, 2013, 13:52:51
Those are black and wet but that is just a place to start from.

How long were they in and how were you riding during that time - hard, nice and easy, redline all the time. How it was ridden is important.

It could be float levels but it could also be a weak ignition on that pair.  I'd start with the points/coils.  Then I'd check float levels and after those are checked/corrected, then I'd look at the air jets, needle jets and needles. But if the problem is just 1-4 and not on 2-3 I suspect weak sparks on that pair.

 
Title: Re: Motorcycle Carburetor Theory 101
Post by: sxecafe on Mar 06, 2013, 14:15:26
I had it rev'd fairly high for about 20 sec and then hit the kill switch. I just pulled 1-4 because they were easiest to get to. What would be wrong with the points coils? I have new Dyna ign and coils.
Title: Re: Motorcycle Carburetor Theory 101
Post by: cqyqte on Mar 06, 2013, 15:09:23
Best jetting post I have read.

However, my bike is giving me problems, and I can't figure out where my problem fits in all of this.

When I bought the bike it had sponge-like unifliter pods on each carb of my 82 CB650.
I installed K&N pods instead.

The bike runs pretty good, i have never ridden a bike of this size so I can't be sure. It idles fine, and seems to have good power. I have gotten it up to the ton no problem with more rpms to spare.

HOWEVER, today I finally put insurance on it, and plated it, and got ready to go on a road trip this weekend and I took it out for a ride. It was moderately windy (more wind than this bike has seen in the short time i've had it). It ran great going with the wind. No problem. When I turned around to go against the wind the bike ran like crap. Whenever a gust of wind hit me the bike bogged down so bad I almost had to pull over. I tried running with the choke half way and it seemed to fix the issue and it ran pretty decent.

So what is my issue? I suppose the bike is running lean since the choke helped.
Why does the bike run fine with lots of power as long as there is no wind?
I want to fix this problem fast for the weekend. I might try to tape up the air filter pods to limit some air and see if that helps.
Which jets would I be looking at changing in this case?

I think I'll post this under it's own subject too.

Thanks!

I has to do with the turbulence that pods cause in the throat of the carb.  The stock air box creates a space of quiet air for each carb to draw from.  Wind direction and gusts have little affect on the air in the stock box, however eliminate the stock box and the mouth of the carb is now susceptable to the wind turbulence as the bike travels down the road.  The stock box also had short smooth tubes that connected the carb mouth to the box, which infact lengthened the throat of the carb creating a laminar flow of air in each carb.  Removing this allows turbulence to play havoc with the drawing affect of the air over the jets (main, and pilot jet opening) changing the metering affect, which in turn causes the air fuel mixture to bounce all over the place.

Now that is a lay person explanation and I am sure there will be others that will jump all over what I said with text book explanations.  I am not here to argue with them, they know better and I admit that.  I have tried some ideas that in some way mimock the OEM air box set up yet still appears as if I am running pods or individual air filters, and they have resulted in more stable situations, but not perfect.

I have made myself velocity stacks that extend the carb horn some 2" out of ABS plumbing couplings.  I have flared the inside throat of the ABS fitting to make a smooth transition using my lathe and then made the opposite end large enough to fit over the carb horn tightly.  Then I went to the K&N filter book and found filters with the opening flange large enough to slide over the ABS fitting.  I then place the ABS fitting on to the carb slid the filter over the fitting and tighten the ring clamp down so the filter and the fitting are held tighly on the carb horn.  The filter completely covers the homemade velocity stack and because the inside diameter of the filter is larger that the outside of the ABS coupling air can be draw through all of the filter material, albeit more easily from the part of the filter that extends beyond the end of the ABS coupling.

A severe cross wind will still cause issues but I have quieted down the turbulence affect significantly.  Adding the side covers back onto the bike would also quieten the air around the pods but then if you are going to run with them why not just go back to the stock air box.
Title: Re: Motorcycle Carburetor Theory 101
Post by: sxecafe on Mar 06, 2013, 15:10:03
Hell of an explanation... and your "Grumpy retired oldtimer... 'eat a Snickers bar'" made me chuckle too.

Lots of info to take in there. I'll do my best to process but the moral of the story is that pods are less then optimal.
Title: Re: Motorcycle Carburetor Theory 101
Post by: Kamn on Mar 06, 2013, 16:03:42
Best jetting post I have read.

However, my bike is giving me problems, and I can't figure out where my problem fits in all of this.

When I bought the bike it had sponge-like unifliter pods on each carb of my 82 CB650.
I installed K&N pods instead.

The bike runs pretty good, i have never ridden a bike of this size so I can't be sure. It idles fine, and seems to have good power. I have gotten it up to the ton no problem with more rpms to spare.

HOWEVER, today I finally put insurance on it, and plated it, and got ready to go on a road trip this weekend and I took it out for a ride. It was moderately windy (more wind than this bike has seen in the short time i've had it). It ran great going with the wind. No problem. When I turned around to go against the wind the bike ran like crap. Whenever a gust of wind hit me the bike bogged down so bad I almost had to pull over. I tried running with the choke half way and it seemed to fix the issue and it ran pretty decent.

So what is my issue? I suppose the bike is running lean since the choke helped.
Why does the bike run fine with lots of power as long as there is no wind?
I want to fix this problem fast for the weekend. I might try to tape up the air filter pods to limit some air and see if that helps.
Which jets would I be looking at changing in this case?

I think I'll post this under it's own subject too.

Thanks!



It can also be the pods blocking off atmospheric ports, finding a pod that has no internal lip can help ...........had this problem with my CX and pods then switched to MikesXS oval pods (no internal lip to block the atmospheric ports)  and problem went away


Title: Re: Motorcycle Carburetor Theory 101
Post by: Kamn on Mar 06, 2013, 16:10:18
like this......

Title: Re: Motorcycle Carburetor Theory 101
Post by: GoshenCafe on Mar 06, 2013, 18:31:57
If he's still running K&n's then I don't think those filters have the inside lip, whereas the cheap emgo pod filters do.
Title: Re: Motorcycle Carburetor Theory 101
Post by: teazer on Mar 07, 2013, 00:12:18
I had it rev'd fairly high for about 20 sec and then hit the kill switch. I just pulled 1-4 because they were easiest to get to. What would be wrong with the points coils? I have new Dyna ign and coils.
  I thought we were seeing 1-4 because they were worse than 2-3.  My bad.

Holding throttle open doesn't count as a plug chop.  The motor needs to be pulling hard at high revs and WOT to get a main jet reading.

It's possible that the pods are obscuring the air jet inlets.  I would check that first and if that's not the issue, try dropping the needles one clip position and see if that helps.  I love stock OEM Honda 4 airboxes.  Inside they contain perfect velocity stacks.  We use those on our race bikes without the box. (different bike and different carbs - they are just a good shape for our application.
Title: Re: Motorcycle Carburetor Theory 101
Post by: DreadRock on Mar 13, 2013, 16:09:57
 thanks for posting this as its very good info to learn from !
Title: Re: Motorcycle Carburetor Theory 101
Post by: Domiken on Mar 13, 2013, 17:07:07
Thank You!  Well done
Title: Re: Motorcycle Carburetor Theory 101
Post by: Qawls on Mar 20, 2013, 04:04:12
Good work thanks a lot!
Title: Re: Motorcycle Carburetor Theory 101
Post by: xxthegonzxx on Mar 26, 2013, 14:14:16
I think I might have found my problem. Thank you!
Title: Re: Motorcycle Carburetor Theory 101
Post by: Derwood on Apr 15, 2013, 04:22:44
GULP!!!!

That is a lot of info for this rookie. I will try to digest as I continue on my quest seek out the answer to which jets i need after removing the air box from my '78 CB550K.

Title: Motorcycle Carburetor Theory 101
Post by: sxecafe on Apr 20, 2013, 13:40:45
Just an update on the continual carb tuning. '74 CB550. I have a flat spot/bog right around 4000 rpm.Off the line and WOT are great. I think it's needle jets, as it's mid-throttle. I have the needle set to stock, which is second notch up, which means to lean it out, if that's what I need to do, I only have one notch left. Is that enough? Thoughts? Could Float level cause that too?


Sent from my iPad using Tapatalk HD
Title: Re: Motorcycle Carburetor Theory 101
Post by: apasqualone on Apr 29, 2013, 15:53:34
if i just did a top end rebuild and my carbs sat for a while, would they be a cause for my bike not to run? them being gunked up that is.

what can i use to get them working well again without tearing them apart? ive heard sea foam. thoughts?

its a 76 hond cb500t btw
Title: Re: Motorcycle Carburetor Theory 101
Post by: roguebuddha on Jun 25, 2013, 02:44:28
pretty sure this is a copy and paste... don't take someone else's work and pass it off as your own please. unless that is your name is Ian Williams or you did this write up for him and his company.... http://www.iwt.com.au/mikunicarb.htm  please show credit!!!!
Title: Re: Motorcycle Carburetor Theory 101
Post by: Hobes on Oct 14, 2014, 00:20:49
should have read this before posting my Please Help me. Despite all my own research i still now know a lot more on BS34 and carbies in general.

Credit due to where it is due.
Title: Re: Motorcycle Carburetor Theory 101
Post by: chucky1970cb100 on May 01, 2017, 21:25:09
*NEW* LINK TO PDF FILE *NEW*
http://www.motorcyclecarbs.com/carbs101.pdf (http://www.motorcyclecarbs.com/carbs101.pdf)
Motorcycle Carburetor Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image001.gif)
FIG 1

Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image002.gif)
FIG 2

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image003.gif)
FIG 3

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image004.gif)
FIG 4

The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image005.gif)
FIG 5

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.

 
(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image006.gif)
FIG 6

Carburetor Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

(http://i929.photobucket.com/albums/ad138/midcoastcustoms/image007.gif)
FIG 7

While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color. If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.

 

Altitude, Humidity, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performance of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32š Fahrenheit may run poorly when the temperature reaches 90š Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found. Using fig 8 as an example, the air temperature is 95š Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

 (http://i929.photobucket.com/albums/ad138/midcoastcustoms/image008.gif)
FIG 8

Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.

Needle Jet/Jet Needle/Air Screw Correction Chart

 

Correction factor     1.04 or above        1.04-1.00        1.00-0.96        0.96-0.92         0.92 or below

Needle jet               2 sizes larger     1 size larger      Same size       1 size smaller    2 sizes smaller

Jet needle setting    Lower clip position    Same              Same               Same          Raise clip one position

Air screw opening   One turn in            1/2 turn in          Same          1/2 turn out      One turn out

All info is from this parts site  http://www.siriusconinc.com/index.php (http://www.siriusconinc.com/index.php)
This is some awesome info. Thanks for sharing.

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Title: Re: Motorcycle Carburetor Theory 101
Post by: 88SS on May 01, 2017, 21:54:21
I needed this so much. Pretty much confirms my suspicions with my carb problem


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Title: Re: Motorcycle Carburetor Theory 101
Post by: stubsryan on May 09, 2018, 11:05:16
Hey I’m curious, if an engine that starts easily without a choke, would that mean when warm the carbs are too rich? The info says the choke function is to make the mixture more rich. Is this the same as using the throttle? I have a Cb350f and they use a butterfly system choke to restrict the air which is fine. I also have a xs750-D that varies that desired outcome differently with an internal system like explained. I don’t quite understand it


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Title: Re: Motorcycle Carburetor Theory 101
Post by: cxman on May 09, 2018, 11:52:02
your xs750 has a enrichener system it helps start by letting the engine suck more fuel in a bypass port whne the lever is pushed down

enricheners are on or off

unlike a butterfly choke that can be partially applied
Title: Re: Motorcycle Carburetor Theory 101
Post by: teazer on May 13, 2018, 12:47:40
The old style Keihin choke plates restrict airflow and that applies more pressure differential across the jets and pulls in  more fuel.

If it starts without choke on a cool day from cold, the primary jet is probably a size too large and will be a little rich when warm.