Throttle/acceleration stutters, some backfiring. Rejet? Suggestions?

[giganticflying]

So, I've been on and off trying to fix this stock 1979 GS850 for about three and a half years now. Previously it sat for four years in a backyard and didn't run. I've gone through all of the conventional work, carb teardown/cleaning/new o-rings, compression test, valve shim spacing, points/condensers, etc.... and now it runs pretty well but stutters on acceleration. Last year someone told me I should just try to run it hard for a couple of trips around to maybe clean out some old carbon that might be gumming things up and honestly it did make a difference but it still stutters a little on acceleration.

Does anyone have any other suggestions? I've been told that it's possible going up one size in jets might be the solution, but I'm wondering if there is something else I'm missing due to inexperience. It definitely seems like a fuel delivery issue. I have done a carb sync with a carb tune, replaced the petcock (with a used petcock so I'm still kind of suspicious on that one but it doesn't leak anymore), replaced air filter and sealed airbox, boots are good, new intake o-rings, float height is good. Electrical seems to be strong and I've timed the ignition.

At this point, I'm sure any advice would help. I just keep hearing the rejetting suggestion and I'm starting to think it might be the next step. Thanks in advance.

 

[giganticflying]

Also, if you think rejetting might be a good next step, where can I find a kit for 79' gs850 vm carbs?

 

[steve murdoch]

Sounds like you have covered most of the usual potential causes.
If you are certain that the carbs are spotless and tuned, do you have an in-line fuel filter? Maybe a clogged vent in the gas cap?
Still got the stock exhaust on the bike?

 

[giganticflying]

I do have an inline fuel filter but just as an experiment I replaced the line with a complete one and the results are the same. I'll check the gas cap scenario.

If I were to go the rejetting route, where would I find jets for the 79' VM carbs? Dynojet doesn't appear to have them.

 

[steve murdoch]

I would contact these guys and explain your symptoms.. http://www.jetsrus.com/
Still it doesn't make sense that rejetting would be required if all the stock parts have been retained.
Please do a follow-up on any new steps taken.

 

[Big T]

At what throttle opening/RPM do you get the stuttering?

All bikes will stutter is the rpm is too low

 

[giganticflying]

Where are the vents for the gas cap? Around the edges? If so they look pretty open to me.

 

[Guest]

Have you tried replacing the plug leads?

 

[giganticflying]

I have replaced the ignition coils and boots. It used to foul plugs pretty regularly but that went away after timing and carb sync.

 

[mavrik]

Are you sure it's in the engine... there are other things that will cause a shutter...
Didn't see the backfire part...

BUMP...

 

[mavrik]

After thinking about it and not really knowing when the shutter occurs I would suggest you move the needles up one position if possible... that should help smooth out acceleration... you don't need to alter the jetting just yet...

 

[Guest]

My go around with VM carburetors with your problems was because they were too rich, fowling plugs in hot humid weather and hitting a wall at about 5500 RPM before stumbling through to red line and a good bit of backfiring when the fowled plugs decided to fire. It was all due to the pilot circuit being screwed up with excessive force seating the pilot fuel needles before I got the bike. In a good set of carburetor bodies the needles when properly lightly seated should be barely visible below the small hole in the throat of the carburetor. If they stick up above the hole they have been stretched out and will be very hard to adjust. I finally scrapped that set of carburetors for a virgin set off a low mileage parts bike and had to soak them twice before I got the small pilot circuits completely clean and working properly. Too lean will spit back through the carburetor and at speed where it peters out it will respond to rolling off and back on the throttle to pass the dead spot. I would look carefully at the pilot circuits and possibly put the carburetors through another 24 hour soak in carburetor dip. An external fuel filter is a problem and you should use the Suzuki fuel line. Thick walled fuel line for fuel injection at the parts store can kink and starve for fuel especially with an external filter. Good luck.

 

[mavrik]

I think at this point you should pull the plugs and let us know what they look like... all 4 of them... that will tell us a lot more than we know now...