Plug Chop help, pictures
- Thread starter djg42
- Start date
djg42
Been Around the Block
Thanks Hillsy!
Will changing the main jet or needle clip postition effect the idle on these carbs? I got the idle nice first, and then after all of the main jet switches and needle clip positions I cant get it right, thinking of moving down on the pilot jet.
Thanks!
Dennis
Will changing the main jet or needle clip postition effect the idle on these carbs? I got the idle nice first, and then after all of the main jet switches and needle clip positions I cant get it right, thinking of moving down on the pilot jet.
Thanks!
Dennis
when they say plug chop, your not actually supposed to chop your plugs.
those pics dont show enough. a short run is not enough to determine off a plug chop. only thing a plug chop is good for is main jet sizing and getting "ballpark." plus to get a good reading you need to be at WOT on for a good minute or so to and then pull in the clutch and kill the engine immediately to get an accurate reading.
those pics dont show enough. a short run is not enough to determine off a plug chop. only thing a plug chop is good for is main jet sizing and getting "ballpark." plus to get a good reading you need to be at WOT on for a good minute or so to and then pull in the clutch and kill the engine immediately to get an accurate reading.
djg42
Been Around the Block
I though just looking at the spark plug nose wasn't reliable? I was using this as a guide,
http://kawtriple.com/mraxl/carb/plugchop.htm
When your trying to pick a main jet, should you use the leanest needle clip position?
The first time I started with the biggest jet I had and worked down, i just left the needle in the middle. But it seems like the needle position even changes the WOT. Everything I try is too rich unless the needle clip is on the top. What does this mean? DO I need a different needle?
Right now I have 200 main jets in at the leanest needle clip, and 1/2-full throttle is pritty good. Its way too rich even on the second to top needle clip position, even if I go down to a 165 MJ.
Any advice would be great, Thanks!
Dennis
http://kawtriple.com/mraxl/carb/plugchop.htm
When your trying to pick a main jet, should you use the leanest needle clip position?
The first time I started with the biggest jet I had and worked down, i just left the needle in the middle. But it seems like the needle position even changes the WOT. Everything I try is too rich unless the needle clip is on the top. What does this mean? DO I need a different needle?
Right now I have 200 main jets in at the leanest needle clip, and 1/2-full throttle is pritty good. Its way too rich even on the second to top needle clip position, even if I go down to a 165 MJ.
Any advice would be great, Thanks!
Dennis
Let's deal with the Plug chop thing first. It's not necessary to chop the plug u[p to see teh results of a plug chop. We use a spark plug reader which is a magnifying glass with in built light that shines down inside teh plug. That way we can see down to the bottom of the insulator.
You are correct that looking at the tip is not how to read a plug - you do need to see down inside and if cutting a plug up is the only way to do it, go for it.
something like this one will do but there are cheaper ones around. Or use a jeweler's loop.
http://www.summitracing.com/parts/CCA-5326/?rtype=10
Now back to the needle.
Normally at WOT, the orifice around a needle is much larger than teh main jet and in that way the needle modulates the main jet at smaller throttle openings and at the top end, it's the main jet that controls fuel flow. In your case, the main jet appears to be way too large still and as the needle lifts, it allows too much of that fuel to flood the motor.
Other possibilities exist. For example a 2 stroke type needle jet allows too much fuel for a four stroke at high speed/WOT conditions even if it's a the perfect main jet at lower throttle openings. A bigger air jet will tend to lean out full throttle/high rev running.
I'd try a smaller main jet with the needle in the center position and start there.
BTW, what bike and carb are we talking about?
You are correct that looking at the tip is not how to read a plug - you do need to see down inside and if cutting a plug up is the only way to do it, go for it.
something like this one will do but there are cheaper ones around. Or use a jeweler's loop.
http://www.summitracing.com/parts/CCA-5326/?rtype=10
Now back to the needle.
Normally at WOT, the orifice around a needle is much larger than teh main jet and in that way the needle modulates the main jet at smaller throttle openings and at the top end, it's the main jet that controls fuel flow. In your case, the main jet appears to be way too large still and as the needle lifts, it allows too much of that fuel to flood the motor.
Other possibilities exist. For example a 2 stroke type needle jet allows too much fuel for a four stroke at high speed/WOT conditions even if it's a the perfect main jet at lower throttle openings. A bigger air jet will tend to lean out full throttle/high rev running.
I'd try a smaller main jet with the needle in the center position and start there.
BTW, what bike and carb are we talking about?
Take it to a dyno shop if possible so they can sample the exhaust under different conditions and get it right.
Get the main jet right at WOT first and then tweak from there. At WOT the main jet is 90% of the story and you can see the air:fuel numbers as the revs rise.
One problem may be that the carbs are huge for that motor so it may be difficult to get right. What needle and needle jet are in that carb?
Get the main jet right at WOT first and then tweak from there. At WOT the main jet is 90% of the story and you can see the air:fuel numbers as the revs rise.
One problem may be that the carbs are huge for that motor so it may be difficult to get right. What needle and needle jet are in that carb?
djg42
Been Around the Block
Thanks! Ill see if theres a dyno place around locally, not sure if I could afford it. Do they test different jets etc..., or just test whatever set up you bring the bike in with?
The needle is 6DH4, ill check on the needle jet. I just chose the Vm34 because it was the same size as the stock carb.
The needle is 6DH4, ill check on the needle jet. I just chose the Vm34 because it was the same size as the stock carb.
Stock carbs are CV type which have a slide that doesn't completely clear the throat and they have a butterfly throttle. Those two factors mean that the airflow through a CV carb is always less than the equivalent Slide carb.
In addition, a bike with a CV carb can stand more or less full throttle from any speed where a bike with a slide carb would stumble and die.
Rule of thumb is to use a slide carb of around 80% of the area of a CV. In this case that would be around 28mm in a slide carb or 30mm in a performance application.
Dyno testers don't all work from the same play book, but most will do a roll on run with gas analysis and allow one or two changes to jets for 40-60 bucks. Call them or drop in and talk to the guys there. What you want to know is where is it rich and where is it lean so you can swap out jets to get it into the ball park.
6DH4 and Q-2 are fairly rich, but the carbs are so big that one might expect low gas velocity and poor pickup needing larger jets and yours is the other way around. Assuming (and that's a biggie) that everything else is spot on, why is that motor showing up as so rich? Is it incomplete combustion leading to unburned fuel going down the tail pipe of is it too little ignition advance causing that incomplete combustion?
A quick run on the dyno with full gas a analysis would help to identify what is actually happening. Don't do it if they have no gas analysis and won't give you any idea of what it might cost.
In addition, a bike with a CV carb can stand more or less full throttle from any speed where a bike with a slide carb would stumble and die.
Rule of thumb is to use a slide carb of around 80% of the area of a CV. In this case that would be around 28mm in a slide carb or 30mm in a performance application.
Dyno testers don't all work from the same play book, but most will do a roll on run with gas analysis and allow one or two changes to jets for 40-60 bucks. Call them or drop in and talk to the guys there. What you want to know is where is it rich and where is it lean so you can swap out jets to get it into the ball park.
6DH4 and Q-2 are fairly rich, but the carbs are so big that one might expect low gas velocity and poor pickup needing larger jets and yours is the other way around. Assuming (and that's a biggie) that everything else is spot on, why is that motor showing up as so rich? Is it incomplete combustion leading to unburned fuel going down the tail pipe of is it too little ignition advance causing that incomplete combustion?
A quick run on the dyno with full gas a analysis would help to identify what is actually happening. Don't do it if they have no gas analysis and won't give you any idea of what it might cost.
200 main jets on a GS450 are way too big. Stock is 115, so you should be around 130/140 max.
And you don't need to cut up your plugs - if you were tuning a 2 stroke to within an inch of it's life, then yes, but you are just wasting plugs and money with what you are doing here..
And you don't need to cut up your plugs - if you were tuning a 2 stroke to within an inch of it's life, then yes, but you are just wasting plugs and money with what you are doing here..
djg42
Been Around the Block
Hmm, well retrospectively i would have went with the smaller VMs! But I have heard of people fitting the VM34s to this bike with good results, so Ill stick with it for a bit longer.
Thanks,My GS had 117.5 stock mains, and before I swapped carbs, after removing the airbox and changin the exhaust, i was using 165 Main jets with good results, but I thought with the VMs maybe I could take it to the next level. It has electronic ignition, compression is spec at 180 on both sides, and I adjusted the valves. I had no problems with the bike when I decided to swap carbs. I read that if you are at WOT and you let of the throttle to 7/8 throttle, its gets a little richer and you can use that to help pick your MJ. Is that true? Because when I tried that with a 180 it still pulled a little stronger when I let of the throttle down to 7/8th. Also, at WOT with 200s its feels insane. How do the hex jets on the VMs compare to the old style jets in the stock mikunis? Are the sizes comparable?
Thanks,My GS had 117.5 stock mains, and before I swapped carbs, after removing the airbox and changin the exhaust, i was using 165 Main jets with good results, but I thought with the VMs maybe I could take it to the next level. It has electronic ignition, compression is spec at 180 on both sides, and I adjusted the valves. I had no problems with the bike when I decided to swap carbs. I read that if you are at WOT and you let of the throttle to 7/8 throttle, its gets a little richer and you can use that to help pick your MJ. Is that true? Because when I tried that with a 180 it still pulled a little stronger when I let of the throttle down to 7/8th. Also, at WOT with 200s its feels insane. How do the hex jets on the VMs compare to the old style jets in the stock mikunis? Are the sizes comparable?
crazypj
Split personality, I fake being smart
3rd gear, slow roll on acceleration, bigger main jet until it miss-fires when accelerating and then come down on jet size until it lean miss fires when accelerating.(it's not as serious as on a two-stroke for a few seconds)
There should only be 3~4 sizes different if motor is in good condition (my 378cc motor goes lean to rich from 110 to 115, 112.5 is just about perfect for stock Kei-Hin with my set up)
An hour on a dyno with EGA will save a massive amount of time (even one or two runs with EGA will probably get you in the ballpark)
Hillsy, different style jets in VM and CV carbs (I found out when I made my own emulsion tubes and everyone else was using 'two-stroke' primary choke tubes)
Probably needs 130~160, depending on exhaust, particularly as carbs are too big for motor
There should only be 3~4 sizes different if motor is in good condition (my 378cc motor goes lean to rich from 110 to 115, 112.5 is just about perfect for stock Kei-Hin with my set up)
An hour on a dyno with EGA will save a massive amount of time (even one or two runs with EGA will probably get you in the ballpark)
Hillsy, different style jets in VM and CV carbs (I found out when I made my own emulsion tubes and everyone else was using 'two-stroke' primary choke tubes)
Probably needs 130~160, depending on exhaust, particularly as carbs are too big for motor