Carb Jet Guru's Pls Help!! 79 GS750

[bhppy101]

Hi all,

I have air pods and a Vance/Hines 4into 1 pipe with VM carbs. I'm getting some light backfire from the exhaust at higher RPM's and its a little sluggish.. I have 130 jets in her right now....what size should I be going with... I'm not sure what the stock jet size was but I know it wasn't 130....Should I go higher or lower on the main jets??

Thanks in Advance!!

[KEITH KRAUSE]

Hi all,

I have air pods and a Vance/Hines 4into 1 pipe with VM carbs. I'm getting some light backfire from the exhaust at higher RPM's and its a little sluggish.. I have 130 jets in her right now....what size should I be going with... I'm not sure what the stock jet size was but I know it wasn't 130....Should I go higher or lower on the main jets??

Thanks in Advance!!

"Light backfire", which I'm assuming is really decel' popping, is a lean condition. The engine is under the most extreme lean condition when the slides are closed but the engine is still at higher rpm's. You can't really expect the pilot circuit to supply adequate mixture under this condition. You generally have to accept some decel' pop. Hard for me to say if your decel' pop is acceptable or excessive. When re-jetting for your mods, you can create worse problems by trying to eliminate all of the decel' pop. You must first be sure the bike is in otherwise good tune and the header gaskets are sealing well. If you're sure, then you might want to try adjusting the pilot fuel screws (underneath) a little richer and test. Maybe an additional 1/4 turn or 1/2 ? Keep records of all adjustments so you can re-set them if needed. If no joy by the time the screws are 3 turns out, then I suggest 17.5 pilot jets and re-set the pilot fuel screws to something closer to factory (about 1 turn) out and test.
Be sure the side air screws are adjusted using the highest rpm method too.
The pilot circuit plug reads should be done after operating the bike at minimal throttle position for a couple miles. A steady 35/40 mph in 4th/5th gear uphill or level is good.
As for the "sluggish" complaint, that's not enough info.
If your main jet is incorrect or not, only full throttle tests will tell. A 130 Mikuni main is certainly "ballpark" if the pods are flowing well and the bike is otherwise tuned correctly. From my experience, pod/pipe mods require increasing the mains from a minimum of 20 to a maximum of 35. It varies from bike to bike. I think your factory mains are 102.5?? So a 130 is in that range. Only full throttle tests will tell. Do what the plugs/performance say.
Don't forget the jet needles. Test at solid 1/3 throttle and read the plugs. If you're trying to make the stock jet needles work, my experience says your jet needles need to be at their richest position (e-clip in the bottom position/furthest from the top) to have any chance of working right. Be sure all factory jet needle plastic spacers are re-installed in correct order. Thicker goes on top the e-clip, thinner goes under.
Also, REMOVE the two floatbowl vent lines and leave the ports open. Fuel starvation will result when running pods with the vent lines attached.
Be sure the carbs are synched with a vacuum tool or you can't accurately judge performance or get accurate plug reads. Valves clearances and ignition timing and side air screws must be set correctly before the synch.

[bhppy101]

Thanks for the tips Keith.... I moved the needle clips all the way to the bottom....now my bike is running way too rich. Plug test shows dry black and sooty.....I am going right now to pick up some smaller main jets 127.5 or 125's to see if this fixes the problem. I'll pick up some new NGK plugs while I'm at it.

I hope this works....the frustration is setting in!

[bhppy101]

I fixed the problem.....

I got some 125 mikuni round jets and turned the mix screw out 1 .75 turns. Runs like a dream with no sputter or backfire at all!

[KEITH KRAUSE]

Thanks for the tips Keith.... I moved the needle clips all the way to the bottom....now my bike is running way too rich. Plug test shows dry black and sooty.....I am going right now to pick up some smaller main jets 127.5 or 125's to see if this fixes the problem. I'll pick up some new NGK plugs while I'm at it.

I hope this works....the frustration is setting in!

Uh, are you mad at me for the jet needle suggestion??
You didn't say what throttle position you tested at. You don't make main jet changes for jet needle related problems. Test the mains at full throttle and the needles at 1/3.
I did read your next post about how the bike is now running better, but I just wanted to mention the above for others interested.

[KEITH KRAUSE]

I fixed the problem.....

I got some 125 mikuni round jets and turned the mix screw out 1 .75 turns. Runs like a dream with no sputter or backfire at all!

Glad it's running well. 125's are a common final adjustment. As I said earlier, depending on the bike, a larger main jet change of 20 to as much as 35 works for your model/mods. So your 22.5 increase is in that range. Sometimes it's trial and error.
I assume the jet needles are in the bottom position?

[bhppy101]

Uh, are you mad at me for the jet needle suggestion??
You didn't say what throttle position you tested at. You don't make main jet changes for jet needle related problems. Test the mains at full throttle and the needles at 1/3.
I did read your next post about how the bike is now running better, but I just wanted to mention the above for others interested.

I'm not mad at you at all Keith...you are probably the best Guru on here! :-D

But another update... I rode today for about 20 minutes and I am having midrange problems.....really flat from 4-7k....when it finally gets into the higher RPM it pulls strong!

Keith....do you think I should move the needles up a notch....and richen my mixture at the pilot?

[KEITH KRAUSE]

OK. It's not about rpm really, it's more about throttle position.
"Flat" performance can be taken either way, lean or rich.
If I had the bike, I'd test and get plug reads at solid 1/3 throttle. That will tell you what the jet needles are doing. If changes needed, make them and re-synch the carbs.
Your "mid-range" descripition could still involve the pilot circuit and even the cut-away, we're talking up to 1/4 throttle position. Again, plug reads after a steady cruise at minimal throttle needed.
What you describe could be anything from pilot circuit to jet needle related. Only way to continue is to separate the jetting circuits with the above tests. If the plugs are darkened after the 1/3 test, clean them well before doing pilot circuit tests. I don't think you said where BOTH the pilot fuel and air screws are right now. Floatbowl vent lines removed? All other tuning done? Carbs synched?

[bhppy101]

I have the mix screws (toward the motor side on top of the bowls) set at 2 turns out.... I'l run a 1/3 throttle test tomorrow....as far as synching.... I am clueless on how to do this....and I don't have a synch tool... I better do some research on synching...

Thanks again Keith!

[KEITH KRAUSE]

OK. Do the tests and that will help you decide what's needed.
It's basic tuning to have all 4 carbs drawing the same vacuum, and thus the same amount of mixture. A poor synch tends to make the plugs burn unevenly and make reads hard to trust. If the reads at all 4 plugs are generally dark, light, or whatever, you can usually tell which way to go with the jetting. But for accurate jetting and judging performance, and quicker/easier jetting, the carbs must be synched.
At the very least, you need to bench synch as closely as possible. You still need to vacuum tool synch but a bench synch is better than nothing.
I posted detailed VM carb bench synch info at a past thread but because I can't remember how to save or send a link, you'll need to search for it. "Hoomgar", a member here, made a technical section thread..."GS1K bad fuel economy/rejet, attn Keith Krause" or something close to that. It's reply #36 at that thread. That should be enough info to help you find it.

[bhppy101]

Hey Keith,

I did the 1/3 throttle test and turned my mix screw in 1/4 turns at a time. I got all the way to 2 1/2 and the bike still stutters and surges at low RPM....Should I go 127.5 on the main jets?

[KEITH KRAUSE]

Hey Keith,

I did the 1/3 throttle test and turned my mix screw in 1/4 turns at a time. I got all the way to 2 1/2 and the bike still stutters and surges at low RPM....Should I go 127.5 on the main jets?

Your description is confusing me. I'm not sure you understand how the 3 jetting circuits operate and at what throttle positions.
You don't make "mix screw" adjustments when testing the jet needles at 1/3 throttle. You raise or lower the jet needles according to 1/3 throttle performance and plug reads.
Also, you need to use the correct terminology so I'm sure what screws you're talking about and if they may be contributing to any stuttering, etc. The VM carbs don't have true "mixture screws", they have separate pilot fuel (underneath/front of bowls) to richen the mixture and side air screws to strictly add more air. With your mods, I'd start with the pilot fuel screws about 1 1/2 turns out and the side air screws set using the highest rpm method, which will usually result in them being 1 1/2 to 2 turns out. Then you test at minimal throttle opening to see what the pilot circuit is doing.
As for the mains, you have to test first at full throttle and see how the performance and plug reads are at that throttle position too. Without specific full throttle performance and plug read results, I can't suggest anything.

[bhppy101]

Sorry Keith....

I have the needle at all the way at the bottom....richest setting.....The air screws are set at 2 turns out now..... as far as the pilot screw....I dont see anything under the bowls at all! No screw is visible

[bhppy101]

Got the carbs off again and took a pic of the bottom of the bowls....is there a metal plug covering the adjustment screw?

[KEITH KRAUSE]

Jeez! The pilot fuel screw is missing. I also see some kind of blockage (charcoal colored area) or something up in there. There should be at least some "sweating" of fuel mixture coming out of the open hole unless there's something jammed in there.
You'll never get the pilot circuit jetted right without the screws and the passages cleared correctly. Without the screws to regulate the secondary pilot circuit passage, this will cause a rich mixture from closed to approx' 1/4 throttle positions. You'll have to be careful and find the exact screw(s), along with their correct springs, spring plate (small washer) and o-ring. The spring isn't absolutely necessary but it stops the screw from losing its setting due to vibration. Sealant would stop movement once you set them right, if that's what you have to do. Getting the springs too is the best thing. Do a search. The screws are available new, but I forget where I saw them sold. If you buy used, beware of getting damaged ones with bent or broken tips. They must be the correct style too. They are actually fairly precision and substituting other style screws will cause trouble.
After seeing this, there could be other things butchered by a PO. I wonder if there are any broken pilot fuel tips in the body, things like that. In that case, you'd be lean on the pilot circuit, assuming everything else is correct. These carbs may need a thorough inspection before continuing the re-jet.