VM CARB TUNING

[Guest]

I HAVE A 1979 GS750L WITH VM CARBS. I HAVE BEEN HAVING SOME CARBURETOR PROBLEMS. MY LOW RANGE RPM IS ALMOST PERFECT ON THE BIKE. FROM IDLE TO ABOUT 3700 ITS GOES, BUT FROM THERE UP, IT STUTTERS A BIT. I CLEANED OUT THE CARBS AND THE JETS ARE REALLY CLEAN. ALSO, WHEN YOU JUST NAIL THE GAS , IT BOGS ALL THE WAY DOWN. YOU CAN ONLY GIVE IT ABOUT HALF THTOTTLE TO GET IT TO GO. IS THERE AN ADJUSTMENT FOR THIS?

[KEITH KRAUSE]

Any exhaust/intake mod's? Plug reads?
List what you've done so far to fix it. Was there any rust in the carbs when you cleaned them?
First thought is a dirty air filter.

[earlfor]

Re: VM CARB TUNING

I had that symptom on my 79 750 whenever the carbs were out of synch.
have you synched it with vacuum guages? Its a must with the mechanical slide carbs even more so than with the CV's.

Earl

I HAVE A 1979 GS750L WITH VM CARBS. I HAVE BEEN HAVING SOME CARBURETOR PROBLEMS. MY LOW RANGE RPM IS ALMOST PERFECT ON THE BIKE. FROM IDLE TO ABOUT 3700 ITS GOES, BUT FROM THERE UP, IT STUTTERS A BIT. I CLEANED OUT THE CARBS AND THE JETS ARE REALLY CLEAN. ALSO, WHEN YOU JUST NAIL THE GAS , IT BOGS ALL THE WAY DOWN. YOU CAN ONLY GIVE IT ABOUT HALF THTOTTLE TO GET IT TO GO. IS THERE AN ADJUSTMENT FOR THIS?

[Tarbash 27]

VM

FIRST OFF ALL, I JUST GOT THE BIKE AND I DONT HAVE AND AIRBOX OR FILTERS. IVE JUST BEEN RIDING IT AROUND THE BLOCK TO TRY TO GET IT TO TUN RIGHT. I HAD THE STOCK EXHAUST ON AND IT STILL RAN LIKE THIS. I CHANGED OVER TO A YOSHIMURA EXHAUST I HAVE AND IT RUNS EXACTLY THE SAME. THERE WAS A TINY TINY BIT OF RUST IN I THINK THE BOWLS OF 2 OF THE CARBS. CARB CLEANER TOOK IT OUT THOUGH. EVERYTIME I START THE BIKE, I HAVE TO PUT IT ON CHOKE. EVEN IF ITS WARMED UP AND BEEN RUNNING FOR A WHILE, I STILL HAVE TO CHOKE IT. BUT ONCE IT FIRES IT IDLES AND RUNS FINE. I AM PRETTY SURE ITS LACK OF FUEL WHEN IT SPUTTERS, BECAUSE OF THE SOUND, ITS JUST NOT GETTING ENOUGH AT HIGH RPMS. ILL CHECK THE PLUGS TOMORROW.

[robinjo]

You need to find a stock airbox or put on pod filters and change the fueling accordingly, you are running way too lean now. My 79 750 went from 102.5 main jet to 116 when running pods.

[KEITH KRAUSE]

As Robin said, you have to decide on what kind of air filter you're going to run.
The bike should be very lean at the moment. If the jet needles are still in their stock position, then they must be raised. Changing the needle positions requires a new vacuum synch, so doing a synch right now will be wasting the effort, unless you know the needles have been raised correctly, which I really doubt. You'll also need other jetting changes, but tell us what filters you decide to run and pipe first.

[Tarbash 27]

vm

WELL I HAVE A YOSHIMURA 4-1 SYSTEM THAT GOES TO A 95 GSX600 KATANA ON THE BIKE AND I WAS PLANNING ON GETTING INDIVIDUAL FOAM POD FILTERS FOR THE CARBURETORS. I KNOW I ADJUSTED A SCREW ON EACH CARB TO GET IT TO RUN BETTER. ITS RIGHT NEAR THE AIRBOX SIDE OF THE CARBURETOR ON AN ANGLE. WHEN I GOT THE BIKE IT WAS 1 AND 1/2 TURNS OUT. I TURNED IT A FULL TURN IN ON EACH CARB. SO ON EACH CARB ITS ONLY 1/2 TURN OUT. THE BIKE RUNS ALOT BETTER BUT STILL NOT PERFECT.

[KEITH KRAUSE]

For starters, those screws, the side air screws, must be adjusted for highest rpm. You can set them now, but re-set them after you've corrected the jetting.
Place the bike on the centerstand, initially set the screws to 1 1/2 turns out. Warm up the bike first. Set the idle to 1,000 to 1,100 rpm's with the idle adjuster knob. Starting at any carb, slowly turn the screw in either direction until you hear the rpm's max. Find the sweet spot. This will usually be between 1 1/2 and 1 3/4 turns out (on a filtered bike). Now reset the idle with the idle adjuster knob to about 1,100 rpm's. Repeat this to the other carbs.
Done.
If you find this doesn't work well for your bike and it insists on running better with these screws only 1/2 turn out, then you have a problem with the pilot circuit.
I suggest getting the filters right now, instead of waiting. It will make things much easier. It's late and I gotta go. I'll try to come back with more jetting help if it's still needed. Get those filters!!

[Hoomgar]

Filtration is good

You'll get er once you get some filters on buddy.

[KEITH KRAUSE]

Tarbash 27, just to add. In preparing for a re-jet, I suggest checking the ignition timing, spark quality, valve clearances. Hopefully, compression is good.
The carbs must be clean, floats checked/adjusted and the o-rings inside good. Also, the manifold o-rings and manifolds must be good. New plugs and exhaust gaskets too. You'll need a vauum synch tool.
If you don't do these basics, then you'll probably end up mixing trouble shooting with re-jetting...not fun. The bike won't run nearly as well either.
A jet kit may be required too. Depends.

[Tarbash 27]

HOW DO YOU ADJUST THE FLOATS? MY CARBS WERE OVERFILLING WITH GAS AND OVERFLOWING OUT THE BOTTOMS, SO I BENT UP THE TABS ON THE FLOATS A LITTLE AND IT DOESN'T LEAK ANYMORE. BUT NOW I HAVE NO BOTTOM END POWER. SO I AM GUESSING I MESSED SOMETHING UP

[KEITH KRAUSE]

You may have just had some dirt or ? stuck in the float valve seats. But you may have inherited floats adjusted wrong too.
You need to get a manual, preferably the factory one. It will show you how to adjust the floats and what the factory setting for them is, among other things. You should use verniers to measure. The float adjustment is fairly precise and is within a hi/low range on your bike of about .08". Yours is probably about 24mm, or .95". That would mean you could set them within a range of .90 to .98", but that's just a guess. Models vary some. You can't just bend the tab and consider it done. When you know the correct setting range, pick the middle of this range and that will give you that little margin for measurement/straight tool error.
With the carbs upside down, bowl gaskets removed, carb body surface clean, floats just resting on good condition float valves, you adjust so your measuring tool just barely grazes the top of the float when you slide it along the gasket surface/channel and over the float. Make sure the bottom of your tool is sliding in the channel, not on top of the small thin ridge that helps seal the gasket from leaking. Make sure the tool is straight up and down. Make sure each side of the float is even before adjusting. Because they are sometimes tweaked, you may have to carefully bend them to make them even. Double check all adjustments.