'78 GS750 / VM29 Troubles

[broquo]

First off, love the forum. You people seriously know your stuff.

Second, sorry for the long write.


Some backstory:

So i was a jeephead who bought this bike two years ago, not running, with 22k original miles without knowing too much about bikes, nevermind the GS line and their pros/cons. Restored the bike a bit, and come to find that this one is the rock solid 8-valve, roller bearing crank, equipped with VM29's and had a 1 up rear sprocket made me boast a smile; wimpy single front disc though. Valve clearance was good, negligible cam wear, dry compression was picture perfect @ 127-135-135-130 but the carbs were totally gummed shut.
I sent the carbs off to wiredgeorge for a cleaning and get them set up simply because i was too much of a baby to try to tune them for pods on my factory baffled megaphone from the 4-1.


When i got them back they were something like #112.5 mains, 1.5 slide, #20 pilot, 0-6 needle jet and a 5F21-4 needle. Never sync'd. It ran pretty well although the whole power band was still lacking nuts. set points, checked advancer, timing, coil re-power mod and it ran a ton better. I know now that I should've done the sync, but i didn't even know what a manometer was then. I drove it sparingly for 2 seasons (>1000mi) running it hot every 2 weeks over both winters with stabil and seafoam in the gas and am currently getting it set for riding this summer.

Since then i know a lot more about the GS thanks to manuals, forums and good old trial and error.

So, last week i decided to finally sync these things so i could adjust the angry little idle and figure out if i needed to go up in jets. The bike had been very difficult to start in the cold, but ran good once hot. Got the carbs all jacked around a few times, but didnt break anything. Took the 5F21 needles (which seem too lean for my setup) to the bottom notch (5, richest) and put the assy back together. Finally got them all zero'd out around the middle of the sync range and got them all pulling 11"/hg @ 2600rpm.

Currently i am running 35mm wire mesh filter velocity stacks, wrapped header, 4-1 megaphone with no baffle soon to be custom 1.75x6" baffle.

Pilot fuel *set @ 1-3/4 from seat.
Pilot air *set @ 3/4 from seat.
(*roughly due to cylinder difference.)

it's really wimpy coming past 1/4 throttle (definitely not burnout capable). She seriously roars a little bit after 1/2 throttle but i think that's cuz the needle is on notch 5 and fully unseated :-) popcorns a little when i close the throttle at any rpm just a little under load to back down the engine speed. Overall gives me the lean feeling...

But it's still hard to start and pops a good bit for the first minute or two until warmed up, so i think it's close to valve adjustment time. Guru Steve hooked me up with his valve shim spreadsheet so I'm in business,


but i know I am in need of some re-jetting soon to fix my weak spots.




So my questions are,
Should i plan on ditching the 5F21-5, #20 and #112.5, and for what? I know all carbs behave differently on different engines of the same build, but is there a generic figure by which to increase main or pilot size to bring out major differences?

Do my carb specs sound even close for a setup like mine?

The SUDCO site recommends the following:
Suzuki GS750
VM29-A12
main: 115
air jet: 0.9
pilot: 25
needle: 5DL31-3
needle jet: 0.6
throttle valve: 2.0


i am also open to ANY useful gs750 and/or VM29 tuning advice as well!!!

*edit* was gonna check plugs today but for the life of me i can't find my plug socket!!

[chuck hahn]

VM 26s ere stock..The 29s ..I think>. are dumping too much gas into the engine. I have the 78 7850 E with the 4 into 1s. Pics are in my albums. I am running the 112 mains with the 17.5 polot jets. The pilot screw on the bottom is at 1 turn from bottomand the air screws on the sides of the carbs are at 1 3/4 out. Of course your bike will be totally different as to where you set the screws. The needle is the 5F21-3 on the middle groove. Bet if you pull the plugs they are all black and sooty.

[Detroit Diesel Man]

Ive got a 77 750 with a Pipe and Pods and I have my Mains at 112.5..needle clip in 1 notch lower than stock (richer) and I run 22.5 Pilots..Fuel mixture is 1 turn out air screw is 1 1/2 turns out...runs hard through every gear and defenitely burnout capable...lol..if you have VM29 carbs I'm with Chuck..too much fuel..if the motor is stock its over fueling it..also be sure your float height is set correctly..carb manifolds dont leak etc etc..


DDM

[KEITH KRAUSE]

I can only assume wiredgeorge cleaned the carbs correctly and serviced any part that needed it. I hope he set the float levels correctly. I won't get into a long story but he makes claims that he doesn't keep. According to several members who have been here he knows little about jetting and the most recent complaint came from a 1000 owner who nearly had a melt down due to george's jetting. I don't trust anything he did.
I'd trust the Sudco site when their jetting parts list differs from what he installed. The cut-away for example regulates mixture flow just before 1/4 throttle and if that's incorrect then no other jetting will work well in an attempt to compensate. Some here have said they used the 1.5 slide while others say the 2.0 works but there's no way both can be right so there's a lot of trial and error when using these carbs.
Personally, I believe these carbs are a little too big for your 750 as it is. That will make it more difficult. One thing I'm confused about is you mention pilot fuel screws. I thought the 29's only had pilot air scews (on the side). Looking at a few diagrams from Sudco and Cyclewarables I see no pilot fuel screw mentioned. Are you sure they are 29's?
I would check a few more sites and get more info on what baseline jetting is needed for your bike but Sudco is supposed to be a good site. My point is there's no use trying to come up with the right mains, jet needle position and pilot jets if you don't have the right throttle valves, air jets and jet needles/needle jets.
If you're sure you're ready then I'd ask for some plug reads. Run the bike at full throttle, chop off and see what the plugs look like. Then run it at 1/3 to no more than 1/2 throttle to see how the jet needle is. Then some minimal throttle to see how the pilot jet is doing. Depending on those readings and performance notes we can try to help. No use guessing if you're rich or lean or whatever at this point. The plugs don't lie.
Be sure the bike is fully warmed up. If you haven't done it yet, adjust the side air screws using the highest rpm method. Use 1,000-1,100 rpm's as a base. Be sure plug gaps are correct and the correct heat range of plug is used. Test on level or uphill, no downhill.
It may end up that you go back to the 26's that came with your bike. The smoothbores ARE a better carb but IF you can tune/jet them correctly you'll only benefit at higher rpm's when their better flow is noticable. If you ride hard then it may be all worth it but the 26's can be jetted much easier and do just fine for most owners.

[broquo]

OK, so a buddy of mine brought by a 13/16 spark plug socket and i brought the 24 beers, and we did some plug chops. i got a circular alley setup, so we ****ed the city neighbors off a bit.

Chuck:
Initial through the ranges plug chop showed rich 1, 3, slightly rich 4, lean 2.

timing showed ok3y idle chops @ load.

I was running B8ES gapped to .035 with dyna 3ohm coils with stock ign.
I regapped to .030.

Was nice slightly rich brown with less black on all plugs. Assume some Pilot tuning required.
#1 showed wet. pilot fuel CW .25 turns. Showed slightly rich. ok. Enriched 2 and set all cylinders 1,3,2,4 for highest idle.

Then we got too drunk to drive the bike and started shooting things...

Carb boots are nice and new. Stretchy new nitrile, even.

I think i'll move the needle back down to 4.


*yes, i believe they are VM29's with pilot fuel screw's under the bowl, @ the engine-most side of the carb . Pic is attached.


WG animosity; He knew more than i did then. Which is why i'm asking questions now. Do it right rather than pay for it. That's why we do this sh*t, right? So,

The range for the throttle valve feels good. once it gets by my poor interpretation of what the pilot should be, it has some brass balls.
Old dude, i'm always ready ;-) plug chops within the next 2 days.
And they do sound too big, but they're what i have and what is cheap and available is good. I do notice a serious romph at high end. I'm thinking the 112.5 might be good, cuz i can get it to 8k in 5th with room to stretch.
So what do you mean by "as it is"? Insinuating i need an 850 kit?

[Nessism]

Those are not VM29's. Most likely stock VM26's...which is actually better for your application I think.

[chuck hahn]

Two things i noticed there..first the plug gap is supposed to be .028 to .030 and second..the point gap is supposed to be .018..too much of both is gonna give you bad spark and over gapped points are gonna fry real fast. Take the carbs off and set the bottom screws at 1 turn out from bottom.Thats rich enough on the pilot circuit. Set all 4 air screws at 1 3/4 out. Regap the plugs and points and recheck the timing AFTER the points are reset.

[TheCafeKid]

Those are not VM29's. Most likely stock VM26's...which is actually better for your application I think.

Ed is right. Not 29s as the drain bolt would be on the bottom of the bowl, not on the side and it would also give you access to the main IIRC. Those aren't VM28 "pumpers" either cause there's no accelerator pump. Look like stock 26s in which case that's a good thing. Easier to tune for your bike.
Dig the bell mouths BTW

[tkent02]

112.5 are not big enough.

[Big T]

112.5 are not big enough.

Ditto

With stacks and an open exhaust, you should be 120 or larger on the main jet

[hampshirehog]

Ditto

With stacks and an open exhaust, you should be 120 or larger on the main jet

And another ditto.

You can set your pilot circuit easily enough with the fuel screw settings that Chuck gave you above. Use the airscrews (on the side on the rear) to fine tune.

Needle position one below middle to start. You may need to go to the bottom one but that's more unlikely in my experience. You can run the thing quite well without any mains in it at all up to about 1/2 throttle like this and it'll feel ok. But I'd start looking at around 120 - 125 for mains

And another confirm that they're not VM29s and most likely to be VM26s - which really suit that motor.

[broquo]

Well, I've looked at oh, two-hundred GS750 pics and either every single GS750 has "VM29's", or these are the....VM26SS? Not a lot of literature out there on these. Those VM29 pics out there are scarce but very close. It's better for me cuz i dont need a $900 set of carbs on a $600 bike
But why does every single VM26 pic come back with an individual throttle cable on every carb?

So stock was 100 mains, 15 pilot 5F21-3 needle.
From a 112.5 to 120 is about a 12% increase in jet hole area. it doesn't quite sound like enough of an increase. The only way to know is try, but would 125 be unwise?
Can i run the 5D31 needle or is there a more-lean richer needle i should be looking for?
bigger than #20 pilot?


Snow and rain cancelled plug chops.

[chuck hahn]

VM 26s are the stock carbs for the 77 and 78 750s. And yes the info you posted on the mains and pilots are correct. turn them around and measure the inside of the throats on the engine side....

[Detroit Diesel Man]

"Then we got too drunk to drive the bike and started shooting things..."

LMAO!!..Thats too funny.

[KEITH KRAUSE]

As I suspected earlier, you have the stock 26 MM carbs.
If the bike is otherwise tuned correctly you can run 125 mains for sure and some 750 owners have used up to 130 mains with good results they say. If you think it runs well now with the lean mains you have then you're going to like what the bigger mains do.
When running the FACTORY jet needles, you'll need to richen their position from 1 1/2 to 2 full positions richer than stock. Half position changes are achieved by using an approx' .022" thick jetting spacer. So IF your stock JN position is #3 from the top, then place the e-clip at the bottom (5th) position for a full two positions richer or leave the clip where it is BUT install a jetting spacer directly on top the e-clip which will be position "4 1/2". Hard to say which will work best without testing. I don't believe one position richer is enough in your case. Be SURE you use the factory jet needle plastic spacers and install in factory order. Thicker one goes on top the e-clip and any jetting spacer and thinner one goes under the e-clip.
17.5 pilot jets should work and you will need to play with the pilot fuel screws underneath. I'm guessing 1 full turn out from lightly seated to start. Richen them if needed (counter-clock) up to 3 full turns. If no joy then you'll need #20 PJ and return the screws to about 1 turn and go from there. I think the 17.5 will work. Make sure you have the correct length PJ's.
REMOVE the two floatbowl vent lines to improve bowl venting and avoid fuel starvation under some conditions (also suggested by Dynojet in their jet kit info.)
With the above jetting installed you need to bench synch the carbs again and follow that with a vacuum tool synch. Adjust BOTH the fully closed throttle position and fully open position (at the throttle pulley stopper screw). Do that after the bench synch. I have detailed info on this site if you search or ask how to do the bench synch and fully open/closed positions. Bench synch is really only to allow the bike to start initially. Be sure you deliberately set the idle screw knob for higher idle so the bike will start. Once started and fully warm, adjust the side air screws for highest rpm again. Then vacuum tool synch. Then test the jets at the required throttle positions. If you need to change the jet needle positions you must re-synch the VM carbs each time.