77 GS 750 ...Jetting almost there..???

[gs750rat]

I just finished rebuilding the carbs and a full tune up on my bike. It is equipped with a 4 into 1 exhaust (unknown tags ripped off: probably Vance and Hines) and I added EMGO Pods (Yeah I know..). I have been been playing around with the jettuing and mixtures and chopping the plugs for a while now. It is equipped with Mikuni VM26's and here is where it is at:

Main Jets: 130
Needle position: Factory needle @ 5th notch, Richest position
Fuel Screw: 2 turns out
Air Screw: 1.25 turns out
Pilot Jet - #15
Idles @ 1100
Vent tubes removed
Float Height: 24 mm measured without gasket
Bench sync'd
Sync'd on bike with Morgan Carbtune ..pulls 22cmhg @1200 ..Above that all stable and even.
This bikes starts ok with a little throttle and choke and returns to idle perfectly when warmed up. I have no backfiring on deceleration. The bike pulls like crazy all the way to redline without hesitation. No dead spots to really speak of.

Chop test results:

Idle: Plugs 1(more than any) and 2 are rich; while 3 and 4 are half tan/black


1/3 -1/2 throttle: Plugs 1 and 2 are fairly rich , but white insulator can be seen with some black still on them. Plugs 3 and 4 insulators are white


Full throttle: All plugs have white insulators with a the steel rim of the plug black.

All cylinders are hot and the bike seems to run A+. I am still curious to why its running so lean at WOT with a 130 main jet. I can't see needing to go bigger. ????? I tryed 125 with the same results previously. It does not seem to be getting better with the larger main jet. It previously has 105's with this exhaust and the stock air box and filter, but always ran rich. I took a lot of advice from this board on jet sizes and settings, tips, and hints.

Will playing with the fuel screws ADD to the mixture up at the top end since the circuits overlap? If so it will only make my idle area richer then it already is no? I could leave it as it is , but am a little concerned on the lean condition causing damage ??? HELP ........Heading to Bike Week this weekend in New Hampshire

Any and all help into getting this lean problem resolved and a full tanning of the plugs all around would be greatly appreciated. Keith and Earlfor are especially welcome. I have read MANY threads on this subject here , but have not pinpointed my bike problem. I can't resolve it. TIA

[chef1366]

With different readings like this you might want to check compression and valve clearance. You can't have more then 10 pounds of difference in compression between cylinders

[gs750rat]

I have checked the compression previously and it is 125-130 psi in every cylinder. The ONLY thing I have not done with this bike is check the valves. The bike runs great, but I'm just baffled by the lean condition at 3/4-WOT.

[KEITH KRAUSE]

Just saw this as I'm going to bed, so just a quick reply for now.
Interesting how 1 and 2 are richer at multiple throttle positions. Did you verify all the jets/adjustments in all carbs are the same?
For now, try 1/2 turn leaner (in) at the 1 and 2 pilot fuel screws (underneath).
Carefully adjust all four side air screws using the highest rpm method.
I've seen '77 to '79 750's run 135 mains.
The valve clearances should always be checked/adjusted BEFORE the vacuum synch. Same with the air screw adjustments. Ignition timing too. Triple check the synch afterwards.
My experience is that the stock jet needles USUALLY don't work well with pod/pipe mod's. I always go with a DJ jet kit or similar.
I'll try to add any more thoughts later if needed.

[gs750rat]

Thanks for your response Keith and may I say that you are a godsend to this board and I enjoy reading your posts. As far as the jetting, I totally striped these carbs down to every single piece and dipped them in berrymans for 24hrs each (choke slide gasket cleaned with keresene). I individually assembled each one( all new rebuild kits) and I know they all have the same jets verified by checking the numbers. I should mention that I installed new coils/wires/plugs and have a nice blue spark from each one. I will atttempt the leaning of 1 and 2 as you suggest and see what happens. Will that not lean me out at WOT more though? The air screw adjustments do not seem to do anything. There is barely a response from any of the carbs when performing this? Any thoughts to why? After reading your posts again I realized the mistake of keeping the throttle open in the tested position and coasting to a stop with the clutch in. I now know I should have closed it at shutoff when coasting to a stop. Would that make a difference? Come to think of it now the bike struggles to start a bit when cold with full choke and after I had moved the fuel screws to 2 turns. I was trying to add a bit more fuel at the top end but maybe i should consider them really ONLY for the idle circuit and not much more of a addition to WOT. Correct? Any thoughts on the temp the cylinders should be around after the WOT chop test to see if it is in a overheat/lean danger condition? I am looking forward to working this through with you and the board. Any other suggests please feel free. Thanks again.

[KEITH KRAUSE]

I totally striped these carbs down to every single piece and dipped them in berrymans for 24hrs each (choke slide gasket cleaned with keresene). I individually assembled each one( all new rebuild kits) and I know they all have the same jets verified by checking the numbers. I will atttempt the leaning of 1 and 2 as you suggest and see what happens. Will that not lean me out at WOT more though? The air screw adjustments do not seem to do anything. There is barely a response from any of the carbs when performing this? Any thoughts to why? After reading your posts again I realized the mistake of keeping the throttle open in the tested position and coasting to a stop with the clutch in. I now know I should have closed it at shutoff when coasting to a stop. Would that make a difference? Come to think of it now the bike struggles to start a bit when cold with full choke and after I had moved the fuel screws to 2 turns. I was trying to add a bit more fuel at the top end but maybe i should consider them really ONLY for the idle circuit and not much more of a addition to WOT. Correct?

OK. Another quick reply as I'm off to work. I'll try to reply here if needed (tonight or Saturday late) but I've been busy at work. Hope you're not waiting on me. I hate "quick" replies.
To try to answer some of your previous questions...
Yes, you must chop off to get the most accurate plug reads. Pilot circuit reads are difficult to test for (road testing) and the results may vary a bit from test to test, but you CAN get a very fair idea of how the pilot circuit is mixing.
Don't be concerned about any overlap the pilot circuit may have on the mains/full throttle tests. Insignificant.
As for your side air screw response, at what point do you reach "best idle"? Has to be somewhere. Basic rule of thumb is if you reach best idle at something less than 3/4 turn out, you're lean on the pilot jet. The "sweet spot" is generally 1 1/2 to maybe 2 turns out.
It figures (because of pod/pipe mod's) the bike is lean on the pilot circuit and that would explain the cold start problem. However, we're getting some mixed signals because you say simple pilot screw adjustments have allowed you to see rich plug reads while testing for the pilot circuit. You also say you have no "decel" popping. Decel' popping is lean condition. It's very common to have to go up to a 17.5 pilot jet when using the STOCK jet needles. But your pilot fuel screws APPEAR (according to first plug reads) to be adding the extra fuel needed to assist the stock pilot jet (??).
As I said before, the stock jet needles often don't work, though the cheaper "Emgo" type pods usually help the stock needles a little. But if they're set to their richest position and you still get lean plug reads, you have to do what the plugs say, IF you did accurate/correct plug testing and the bike really is tuned right and ready to be re-jetted.
Same for the main jets.
As for those mains, are they for sure Mikuni jets?
Sorry to have to rush but I gotta go to work. I still have some questions/ideas but I'll have to reply later tonight if you still need help.
I'm sure we can figure it out. I'll be assuming that ALL other tuning has been done right, especially float levels (I have to wonder if they're set right/.94" is best).
By the way, valve clearances are an important part of the tuning. I'm not saying this is your problem, but it can be a contributor to problems.
Talk to you later.

[Hoomgar]

Hey buddy. What's your first name? I don't want to be calling you rat

I am in no way going to try to supersede Keith as that is where most of my jetting experience has come from. However, I have a good deal of experience with the same carbs that you are using and the same problem. Keith helped me resolve it and in his time absence I will share with you what he helped me do. I sure as heck am not going to ask you to read my 33+ page tech post on jetting my vm26ss carbs! If you have already, God bless you! You may need glasses.

Anyway, the issue you’re having is all too familiar to me. Had the same issue. I am going to echo Keith and tell you that you really need the check your valve clearances before you go any further. If you have discrepancies there you will be wasting your time and Keith and well, everyone’s time until they are set right. You cannot tune around a valve clearance issue. You can only compensate for it and that is not good.

After you have checked/adjusted your valves make sure your timing is spot on. If it is electronic ignition upgraded you are most likely OK but someone may have some tips for you there as well. I am familiar with points and if that is what you have then make sure your timing, gap and dwell are all spot on before you continue.

If you had to make any adjustments from those first two then it is time for another vacuum sync. And then new plug reads.

Now assuming you were either OK on all the above or have done it all the next thing is this. Get a DynaJet stage III jet kit now! Save yourself from countless hours and days of needless aggravation my friend. Those stock needles are good for stock air boxes and pipes but with Pods and a 4into1 you really do need the DynaJet stage III kit. Yes stage III is right. It will be money well spent. This alone may be your only issue if everything else pans out.

Lastly, I know it is easy to suggest spending money when it is someone else's money but I assure you that you will have way better, more permanent results if you get some K&N Pods for those carbs. The Pods you have will work but don't last and vary the performance of the jetting greatly as they age. They do not clean well and need constant replacement. The K&Ns will pay for themselves in more ways than one over time.

I hope some of this helps? I do know that the advice I am giving you is from the experts and is time tested by me so I am only an echo of the masters here and cannot take any credit for myself.

It's a lot I know but it appears you already have a good grip on what is going on here so I suspect you won’t have much trouble with any of this.

Rock on! 8)

[KEITH KRAUSE]

Good info Mark.
Rat, you say you've read some of my past posts so you know I always say the bike should be completely tuned/prepared and THEN jetted. It does take some work and some extra expense and sometimes some of the checks don't turn up anything wrong, such as valve clearances, timing, etc, but it DOES make most re-jets easier/quicker in the long run. It doesn't take much to have problems when tuning 4 cylinders as closely as possible. Any little thing can cause trouble and have you scratching your head. Your uneven reads are nothing new. Lean plug reads are just that....lean plug reads. If you've gone through the bike completely and know your work is good and all parts are good, then do what the plugs/performance tell you to do. Yes, at some point certain changes may not make sense or be "typical". Having to make unusual changes may mean something else is wrong. Just don't mix jetting with troubleshooting. Check everything first that effects jetting and can cause variations between cylinders.
If you want to try to make those pods work and want to try the stock needles, we can always try. It's your time. Going back, you did get two cylinders burning on the rich side and two were lean while testing the jet needles. Problem is, you don't know which reads are truly related to poor jetting and which are related to poor tuning or bad parts. Something unknown is causing this. You're pretty sure you've checked the things that would at least give you consistant reads or you wouldn't have put everything back together to test. But you didn't get consistant reads, good or bad, except the full throttle test appears to be all equally (?) lean. Your compression is good, vacuum synch is good you say, floats adjusted, the carbs clean, all jets identical, good spark.....but the timing, dwell (if points), valve clearances need checking. I always replace the manifold o-rings and inspect manifolds as part of any carb service. The pods could have been damaged by a PO simply by cleaning them wrong. For example, you'll compromise the flow of K&N's simply by hose rinsing from the outside-in or using air pressure to dry them. Yours may be new but I just want to point out unknowns. You could have some worn jet needles or needle jets, or worn/weak float valves. Many things can cause uneven reads.
Your uneven reads concern me more than the lean main jet reads. Once you have basically equal reads, rich or lean, at least you know what to try next. With your jet needles at their richest setting, you have two rich plugs and two lean plugs. Could be many things. You need to eliminate anything that can cause trouble. So you need to complete your tuning first, then focus on the jetting.
The only jetting adjustment I've suggested so far was on two of the pilot fuel screws to see if you could even up the plug reads, but let me back up and stress the tuning above first. Then new plug/performance tests. If the stock needles just don't work, then the DJ kit should fix you up.
If you still need help, let us know. Please answer all questions asked already. Add any info or other symptoms you can think of.
Hope you enjoy Bike Week.

[gs750rat]

Keith and Mark. Bike week was great. I will gather all the answers to your questions and reply so we can nip this thing. The bike went 700 miles this past weekend without major issues. I will get back to you ASAP as I am at work currently. Thanks so much for your help so far, the world needs more people like you guys.
Al