Under deceleration what could be causing this..

[onchiman]

I'm still struggling with ironing out a problem with my '79 GS850. Basically this is what is happening so I'm looking for some ideas. Engine runs fine, starts and idles well, pulls smooth and hard through all ranges. When the engine is warm, after deceleration in gear, stop at a stop sign, then go to proceed, it sputters. I have to "clear its throat" like the Harley riders do. My theory is that somehow during periods when the engine vacuum is high, something is happening, like it's pulling in extra fuel or air. The plugs look fine, nice tan color. I also get some mild sputtering while in the low gears trying to maintain a slow speed. Once the rpms are over 3000, no sputtering and all's well. I've tried everything I can think of. I can't tell if it's one cylinder or more involved in the sputter. Bike has 40,000 miles. Electronic ignition, clean carbs with new o-rings, new intake boots and o-rings, new air box boots, air box sealed with new uni filter element. It's all stock setup except for the Dyna S ignition. What could be happening to cause this cursed problem?

 

[Nessism]

I've experienced this issue on my VM carb equiped 550. I'm not sure but it could be related to a high float level spilling over while braking. I'm going to check float height with a sightglass like the service manual shows. Couldn't hurt.

 

[onchiman]

I did check my float levels with a clear tube and adapter in the bowl drains. All looks fine levels in spec. Seems like the engine vacuum under deceleration is causing something to happen. The valves are all adjusted too by the way. I was wondering if somehow the cam timing was off if on the intake stroke would it push the fuel charge back out into the carbs then on the next stroke make the mixture too rich? I'm just tossing out ideas at this point.

 

[Nessism]

How about installing some new plugs and doing a plug chop after running in the "sputtering range" for a couple of miles or so? Find some place where you can really focus on the offending range to give the plugs some color - to tell you if it's rich or lean in that situation. Once you figure out if it's rich or lean you could try adjusting the mixture screw on the bottom of the carb since that controls low speed mixture to some extent.

Not sure this helps...just thinking out loud.

 

[SPARKSS]

...Engine runs fine, starts and idles well, pulls smooth and hard through all ranges. When the engine is warm, after deceleration in gear, stop at a stop sign, then go to proceed, it sputters. I have to "clear its throat" like the Harley riders do. My theory is that somehow during periods when the engine vacuum is high, something is happening, like it's pulling in extra fuel or air. The plugs look fine, nice tan color. I also get some mild sputtering while in the low gears trying to maintain a slow speed. Once the rpms are over 3000, no sputtering and all's well. ...

I've keyed on the description above. Does it sputter when cold, when riding slow (and how slow......rpm....is this happening). Based on your description it does sound heat related and if it were mine, I'd want to see whether you have a carb-head leak when hot - first. Run it to temperature and stop (idling)....I often use an unlit propane torch as a clean pinpoint fuel source to look for leaks (it'll either idle up if lean or down if rich already). Irregardless whether you see a change in idle, keep some propane going and throttle up slightly to see whether the problem is now covered up (you might have a good neutral mixture but still have a leak).

Using this trick into the airbox (filter in or out) can help determine if it's a fuel related problem as well (without any leaks).

Based on the history of the problem, and what I'd determined above, I'd troubleshoot further or see a fix. Another thought......if it's quite hot where you ride, AND you're running regular or perhaps ethanol blend gasoline, try a tank of high octane and see if the problem improves or disappears. This may indicate that you have a fuel percolating problem during hot conditions (which I've seen helped/corrected as per my suggestion).

 

[gremlin]

sounds like you have a bit of gunk in the carb. check and see if all the pipes warm up at the same speed. when you stop the gunk moves, when you take off it clears. its a bear to find.

 

[KEITH KRAUSE]

If carb related, a problem (as you describe) that shows up only when warmed up suggests a rich mixture.
Did the bike suddenly start doing this or did you buy the bike as it is running? Simple pilot fuel screw adjustments may be all that's needed UNLESS it happened suddenly.
Could also be the ignition timing advance is not operating smoothly or the timing is off. Could also be a weak spark/low voltage problem that improves somewhat as the rpm's rise. How much voltage at the battery before start up and also at approx' 5,000 rpm's?

 

[onchiman]

I've checked the timing and it's spot on. The advance works as it should but the springs seem a little weak, it really easy to move the advance. I don't think this would cause the trouble I'm experiencing. The trouble appeared this summer not too long after I changed the exhaust. The original pipes rusted out so I purchased a set from a 1981 model. The only difference in the exhaust is that it has a crossover whereas the original '79 system did not. The plugs indicate a perfect mix on 1 & 4 and perhaps a touch rich on 2 & 3. I made sure the intake boots and o-ring surfaces on the head are sealing properly. I agree with Keith, the trouble seems to say rich mixture. The pilot fuel screws are 5/8 out and air screws adjusted for highest idle. Would it be advisable to go in on the fuel screws on cylinders 2 & 3 to say 1/2 turn or less? I don't want those middle cylinders to get too hot.

 

[Steve]

I don't want those middle cylinders to get too hot.

If you don't want them to overheat, leave them rich. :shock:

A lean mixture burns hotter.


.

 

[walt]

It's a PITA, but do the sight glass test both with the motor running and off. Also never raise the measuring tube. It can be lowered but not raised or you get a false read. You may show different readings with the motor running. Running is the better test for what is really happing.

 

[KEITH KRAUSE]

I've checked the timing and it's spot on. The advance works as it should but the springs seem a little weak, it really easy to move the advance. I don't think this would cause the trouble I'm experiencing. The trouble appeared this summer not too long after I changed the exhaust. The original pipes rusted out so I purchased a set from a 1981 model. The only difference in the exhaust is that it has a crossover whereas the original '79 system did not. The plugs indicate a perfect mix on 1 & 4 and perhaps a touch rich on 2 & 3. I made sure the intake boots and o-ring surfaces on the head are sealing properly. I agree with Keith, the trouble seems to say rich mixture. The pilot fuel screws are 5/8 out and air screws adjusted for highest idle. Would it be advisable to go in on the fuel screws on cylinders 2 & 3 to say 1/2 turn or less? I don't want those middle cylinders to get too hot.

If the problem turned up without any previous pilot fuel screw adjustments, then I wouldn't adjust the screws. That would just be an attempt at compensating for the real problem, which never works out well.
5/8 of a turn out is about the minimum the factory sets these screws on your model. In my experience with jetting changes, leaning the screws another 1/8 or even 1/2 turn isn't going to be the cure for a "sputtering" problem as you describe. I'm considering the low rpm that the problem goes away and the very small change in throttle position. It just doesn't sound like the typical pilot fuel screw problem to me. I've made similar changes during re-jetting and never experienced sputtering by over-richening these screws too much, certainly not 1/2 turn, which is nearly the limit you have with your screws. The bike at one point did not stutter with the screws right where they are, so I'm against adjusting them.
If carb/vacuum related and that non-stock crossover pipe factors in here, I'd suspect a poor vacuum synch with the new pipe and/or it's out of synch from normal use. Regardless if it's the problem or just a possible contributor to the problem, a vacuum synch should be done as part of normal maintanance anyway. Carbs can be out of synch without creating a sputter but yours could be way off so it's worth doing.
Many models with stock exhaust require that you adjust the 2/3 vacuum just a little lower than the 1/4 carbs. Approx' 3/4" (?) less on the typical vacuum gauge such as a Carbstix or Carbtune. With you changing the exhaust, I'm not sure what the optimum levels are best for your change. I doubt that the different pipe is actually causing the stutter because it effected the vacuum that much but it's possible. I feel that the synch may just be poor itself and needs checking.
Beyond that, I think you may have a weaker spark at 2/3 and that coil should be tested. Depending on what tuning tools you own, it may be best to test the coil first.

 

[onchiman]

Well after going round and round with this I did something rather counter-intuitive. I originally had set the pilot fuel screws at their baseline, 7/8 open and then adjusted air screws for highest idle then began adjustments from there. No joy, then I decided to instead open each air screw 1 1/2 turns and then adjust each pilot fuel screw while the engine was running to achieve the best idle (burnt the heck outta my knuckle though). Each cylinder was a little different I then tweaked each air screw (none needed really much at all) for highest rpm and low and behold no more sputter, the symptoms are much improved. It got dark but I'll do a vacuum sync and some plug chops to dial it in but expect that will cure the remaining slight hesitation. Thanks Keith, I got thinking about the pilot circuit and fuel screws and how they interact with one another. For some reason I never considered setting each air screw to the baseline then the adjusting the pilot fuel screws while running. Probably because I don't have a right angle screwdriver so had to reach under the carbs with driver bits and adapters.

 

[KEITH KRAUSE]

Well after going round and round with this I did something rather counter-intuitive. I originally had set the pilot fuel screws at their baseline, 7/8 open and then adjusted air screws for highest idle then began adjustments from there. No joy, then I decided to instead open each air screw 1 1/2 turns and then adjust each pilot fuel screw while the engine was running to achieve the best idle (burnt the heck outta my knuckle though). Each cylinder was a little different I then tweaked each air screw (none needed really much at all) for highest rpm and low and behold no more sputter, the symptoms are much improved. It got dark but I'll do a vacuum sync and some plug chops to dial it in but expect that will cure the remaining slight hesitation. Thanks Keith, I got thinking about the pilot circuit and fuel screws and how they interact with one another. For some reason I never considered setting each air screw to the baseline then the adjusting the pilot fuel screws while running. Probably because I don't have a right angle screwdriver so had to reach under the carbs with driver bits and adapters.

Hmmm. Never adjusted the screws in reverse but if you say it worked for you...
I don't see why you couldn't get the same results with the standard procedure. Easier to do in my opinion and easier to make changes if you need to when cooler/hotter weather takes over. The pilot fuel screw o-rings will leak if disturbed too much.
I'm a little surprised that only 1/4 turn was the cure (or cause) for the stutter. Of course, we can all interpret "stutter" differently. I've made much larger changes to the screws without creating or eliminating a stutter. To go from 5/8 to 7/8 and cure the problem does surprise me, especially when you said the bike only did it when warmed up. As the motor warms up, the conditions richen. That's why choke isn't needed on a warmed up motor. Your stutter, as described, certainly pointed to a rich condition. You richened the mixture and it helped. Strange to me.
Glad it worked though and thanks for sharing.