Jet size and exhaust changes

W

womanisadangercat

New Member
Greetings,
I picked up a 73 Honda CB500 late last summer via Craigslist. Since then I have been working on getting her road-worthy for the upcoming riding season. Currently I have velocity stacks installed and some ugly looking straight pipes that a previous owner put on. A couple things have happened while my rebuild that require a little more expertise then I have. Long story short the previous owner said that he had cleaned out the tank and put a protective coating on the inside (not sure what). Also he said that he had just cleaned and synced the carbs. She ran nice for a few weeks before cold weather set in and I started tearing into it. I recently reconnected all of my electrical components and fired her up. After checking the bike over I noticed that gas was pouring out between one of my carbs and the float bowl. After this discovery I took the carbs off and gave them a thorough cleaning, following the instructions laid out in http://www.motorcycleclassics.com/mc-how-to/honda-cb500-carburetor-rebuild.aspx as well as the Clymer manual. Once cleaned and re-installed I started her back up. I got some slight smoke coming from an unknown location and am willing to bet the Carbs are decently out of sync. Would this slight smoke that I am getting possible be a result of un-synced carbs?
Here soon I would like to ditch the straight exhaust I currently have and go with a Mac 4-in-1 system. If I need to re-sync the carbs and if I am going to put a new exhaust system on I am assuming I need to rejet also. Does anyone have a suggestion of where to start with jet size if I were to run with velocity stacks and a mac 4-in-1 system (possible without baffles)? I’d like to just knock all of this out at the same time to save myself more work in the long run. I apologize for the long explanation that I have written but I know that thoroughness is a must in these forums. Any help that can be given will be greatly appreciated!

I have attached a picture of my bike from a few weeks ago for reference...
 

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Before people start at you, I dont see the front fender on there. Did you leave the bracing from the fender above your tire on your shocks? I cant tell but it doens't look like its there from the pic. You will want that if you plan to be doing any sort of turns.
 
A couple of helpful carb resources for jetting, etc that I've found useful:

http://www.sohc4.net/cb550-faq/indexphp/cb500550-carb-specs

http://www.motorcyclecarbs.com/Motorcycle_Carburetor_Jetting__W7.cfm

http://www.4strokes.com/tech/howtojet.asp
 
Smoke can be multiple things, if it smokes just on startup, it could just be a valve seal leak. What color is the smoke, how long etc. Another reason a bike would smoke, is condensation build up from sitting/not running the bike long enough to get hot enough to evaporate the condensation in the exhaust and engine itself. This is why you don't just "fire" a bike up in the winter to keep the carbs clean...it destroys your ride from the inside out! Is it missing both rear shocks in that picture, as I don't see a monoshock...
 
Thank you all for the input so far!

JSJamboree - I took of the front fender off for the time being because it was in need of a cleaning. I'm also thinking about putting a shorty front fender on but its all about priorities at the moment. I appreciate the heads up on that though.

canyoncarver - I appreciate the links that you have supplied. I have already found some good information from them!

MotorbikeBruno - I had a feeling that the smoke was coming from fuel that was spilled on the engine an burning off. I gave it a good wipe down this evening and started her up to see if it made any difference. After idling for about 5 minutes and struggling to find any smoke, I finally spotted some coming from the LH side intake area. This was the side where the carb leak happened so I maybe some fuel spilled in this area and is burning off? I took a picture so you can check it out. There is definitely crud there that I need to clean off.

The bike is currently idling with no issues at about 1,500 RPM but if I quickly roll on the throttle its starts to bog down and will die if I don't let off the throttle. I can ease into a high rev but it's a no go if I roll on the throttle quickly.

Any thoughts/suggestions?
 

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I forgot to add that in the first picture I had the rear shocks off so that I could give them a good cleaning.... No mono shock, I'm not at that skill level of fabrication quite yet...
 
Well... Sounds like it could be a number of things. Its possible that its running rich (maybe) or the carbs are in need of a sync. What filters are you running?
 
Did you set the mixture screws correctly? More air means you need more fuel. Sounds like your idle is "fine" but when you roll on, it's most likely stumbling from not enough gas if you haven't changed the mixture screw (or upped the pilot jet) to compensate.

Another thought, is that if you didn't put new o-rings on the main jets, too much fuel can be pulled into the carb throat....


Like VonYinzer asks, what jet sizes are you at now?
 
I just tore into the carbs last weekend and I do know that the main jets were 100s. I do not know what the pilot jets are without taking a look. I may not get to that until this weekend (gotta give the broadzilla some attention). Air screws are currently turned out 1.5 rotations. I see that the standard should be 1 plus/minus 1/8th rotations. I'll have to adjust and see if that helps at all. Finally no, I did not change out the o-rings on the main jet when I cleaned them last weekend. From all the research I have been doing I really think that I am going to do the following..

I am really looking into a mac 4 in 1 system which would require a rejet from what I have found. If I got this route and with the velocity stacks that I have already installed, I would have to go up three steps in main jets and 1 in the pilot jet. **please correct me if I am wrong but main jets go up in size increments of 5 and pilot jets in increments of 2.5?** That would mean I should purchase size 115 mains and 42.5 pilots. I would like to replace the orings on the jets anyways and the also replace the jet needles (i just don't trust the old ones I have). I have a mechanic friend who also has a motorcycle tech degree and is willing to help with a carb sync. Any thoughts or recomendations?

Once again I appreciate all the input! All of your experience has been very helpful so far!
 
You are correct on the jetting increments. And with free flowing exhaust and intake, you are going to be about right on the jetting too. That's not to say you won't have a dip somewhere in the throttle curve, but it will most likely be ridable. You will most likely change jetting a few times, so get ready for that and yeah, get some new O-rings...cheap insurance to know they are OK right?

I believe you have the stock components in the carbs as I am pretty sure 100 mains are stock. So we can assume for now that your pilots are stock too. What's wrong with the needles? Crooked? Pitted? worn looking?
 
Sorry, wrong word usage. I meant to say that I want to replace the float needles, not the needle jets. The needle jets looked fine when they were out last. I have heard that it is hard to determine if a float needle is not functioning properly so if I am going to get parts to rejet I might as well replace the float needles while I am at it.

I think that I have most things figured out at the moment. Looks like I will be making a few purchases tonight. I’m going to go with the 4 in 1 exhaust, get some new orings (cheap insurance is fine with me), 115 main jets, and 42.5 pilots. Maybe look into sync gauges because I assume I will be using them more than once.

I know there are multiple places to get jets, orings.. etc. Anywhere you would suggest in particular?
 
i would do this. based on jets being stock. if not find out what the stock jets were and figure out where you are on the equation.

+4 jet sizes for straight flow no baffle exhaust
+4 jet sizes for pod filters.
now add up all the main jet size increases and subtract 2.
and also decrease main jet size by 2 per every 2000' above sea level you are.
and for your pilots add 1 size up per every 3 main jet sizes and decrease by one every 6000 feet above sea level.

this has been the method that has worked for me and has worked out great on the first try usually. BUT remember that jetting is a delicate procedure and may require a lot of trial and error.

and carbs are like food recipe's, everybody has their own opinions and methods, this is simply the way i do it and has worked. others may disagree.

once jets are in, close your mixture screws and open half a turn at a time until you hit the sweet spot.

check out jetsrus.com and dime city for hard to find pilot jets. they are my number one go to place.
 
hurley209 said:
i would do this. based on jets being stock. if not find out what the stock jets were and figure out where you are on the equation.

+4 jet sizes for straight flow no baffle exhaust
+4 jet sizes for pod filters.
now add up all the main jet size increases and subtract 2.
and also decrease main jet size by 2 per every 2000' above sea level you are.
and for your pilots add 1 size up per every 3 main jet sizes and decrease by one every 6000 feet above sea level.
this has been the method that has worked for me and has worked out great on the first try usually. BUT remember that jetting is a delicate procedure and may require a lot of trial and error.
and carbs are like food recipe's, everybody has their own opinions and methods, this is simply the way i do it and has worked. others may disagree.
once jets are in, close your mixture screws and open half a turn at a time until you hit the sweet spot.
check out jetsrus.com and dime city for hard to find pilot jets. they are my number one go to place.
Click to expand...

So to be helpful: +8 jet sizes, subtract 2 = +6 jet sizes assuming 100 is stock jet, new jet size should be 130's and subtract one more (for 900ft above sea level) to give us a total of 125 Main jet size!

Pilot goes up 2 sizes as 40 is stock that means 45 pilot jet

Stock settings can be found here: http://www.motorcycleproject.com/motorcycle/text/specs.html

That look right Hurley209? I'm enjoying this as I've got my 71' CB500 just coming together after valve grind and such. ;)
 
That is a huge increase. with stock carbs and pod filters plus a 4 into 1 pipe your starting point is going to be somewhere around 110 to 115 I would think.

But the place to start is with clean carbs that are synched correctly. Then get the pilot jets right and only then start to think about main jets.

All that crap on the intake manifolds suggests an oil leak, but it is just as likely to be smoke from an engine breather hose. They are usually connected to the air filter box and when that's removed, the hoses are often left dangling.

I would put any velocity stacks on the shelf unless you like to rebuild motors ever couple of months because they wore out. On the street you need decent air filters. Opinions seem to vary between good K&N and UNI filters but in both cases the larger the better. If you can get large filters to fit over yopur velocity stacks, that might be a nice combo solution.
 
I thought that seemed a tad high. And from what I learned, the early CB500's /550's had a larger pilot from the get go, which helped with the takeoffs from a dead stop. I ran 110's on my 1976 CB550 and it pulled great from 1/3 on up. Never really did get that low end perfect.
 
And that's a really good point. Although the best idle often comes with a small jet, to start to accelerate the bike needs a richer mixture as you roll on the throttle and the solution is often to jet richer than ideal at idle to get a smooth roll on.

It's not necessarily ideal, but sometimes it's all that works. In this case, teh OP needs to start from scratch and get one thing right at a time starting with cleanliness and synch after he fits a decent exhaust and filters - no point in doing it before.
 
what i posted was in regards high flow everything Bruno, it would be too rich if youre running a stock airbox and baffled exhaust.
 
hurley209 said:
what i posted was in regards high flow everything Bruno, it would be too rich if youre running a stock airbox and baffled exhaust.
Click to expand...

Right, I thought the OP was going to a pretty open exhaust, and velocity stacks. It would be insanely rich if it was stock airbox and exhaust! ;D
 
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