Stumble at initial throttle blip...

[hikermikem]

I've gotten my '81 GS750L running very well...

Previous posts here describe all the work, suffice to say all the bases have been covered (cleaned carbs, adjust valves, check ignition, etc...)

The only "issue" I'm having is when the bike is all warmed up & at idle (like at a traffic light), it stumbles a bit when you twist the throttle. It will even stall out if you blip the throttle hard & quick. Once past 2 -3K RPM it revs fine, no hesitation.

I'm still playing with the mixture screws. When I set them per the highest RPM method they wanted to be between 2.5 - 3.0 turns out. However, the plugs looked quite lean at that setting so I kept turning them out in 1/4 turn increments until the plugs looked nice & tan/light brown. That resulted in the mixture screws out between 3.75 - 4.5 turns.

All stock except for the K&N drop in air filter; raised the needles .055" (2 washers) & the the mains are oversized one size. I initially raised the needles .032" (3 washers) but it still had a little flat spot around 4500 RPM.

I haven't yet done plug chops at other throttle positions, but I don't expect problems. Acceleration through the jet & main circuits is strong & smooth.

I've synced the carbs a couple of times (VERY touchy) at the earlier settings (best RPM settings), but haven't yet re-synced them with the screws out at the current 4.0+ turns out. I'm hoping a re-sync now will smooth out this transition stumble.

Looking for any insights or suggestions to eliminate this off-idle stumble. Other than this one issue, the bike runs VERY well & I'm quite pleased with the overall perfomance. Even the OEM suspension results in decent handling, (better than my GS850GLZ with the progressive front springs).

TIA,

Mike

[Larry D]

I had a similiar problem Mike. Mine ended up being that the pilots were to large. I replaced them with 42.5 and the bike screams off the throttle now. My idle screws are set to 2 3/4 turns out. I haven't messed with the needle position, but, the bike is running fantastic now.

Edit: After the bike is warmed up, and you blip the throttle....if it dips and wants to die the idle screws are to far out or rich. Under the same circumstances, if the idle hangs and won't settle then the idle screws are to far in or lean.

[hikermikem]

I had a similiar problem Mike. Mine ended up being that the pilots were to large. I replaced them with 42.5 and the bike screams off the throttle now. My idle screws are set to 2 3/4 turns out. I haven't messed with the needle position, but, the bike is running fantastic now.

Edit: After the bike is warmed up, and you blip the throttle....if it dips and wants to die the idle screws are to far out or rich. Under the same circumstances, if the idle hangs and won't settle then the idle screws are to far in or lean.

Hi Larry - thanks for the response. My pilots are 42.5, that's the stock size...

I will play with the idle screws again, I think you're right about them being the culprit. Originally, I had them set at 2.5 - 3.0 turns (using the highest RPM method) but the plugs looked lean. As I stated, I set them to around 4.0 turns out to get decent color on the plugs.

I'll start over again & re-do the idle screw adjust & sync.

Besides, it's fun to play with my Carbtune!!
Mike

[tkent02]

I will play with the idle screws again, I think you're right about them being the culprit. Originally, I had them set at 2.5 - 3.0 turns (using the highest RPM method) but the plugs looked lean. As I stated, I set them to around 4.0 turns out to get decent color on the plugs.

Aside from the plug colors, how did it run when the screws were set to highest RPM?

[Nessism]

Hey Mike,

I wouldn’t attempt to affect plug color though pilot screw tweaks. Set the screws to the highest idle setting and focus on getting the bike to run properly. After you get everything sorted then do some plug chops to judge color.

[hikermikem]

Aside from the plug colors, how did it run when the screws were set to highest RPM?

Ran OK, seemed a bit lean though, some popping on decel...

[hikermikem]

Hey Mike,

I wouldn’t attempt to affect plug color though pilot screw tweaks. Set the screws to the highest idle setting and focus on getting the bike to run properly. After you get everything sorted then do some plug chops to judge color.

Yeah, maybe I'm reading too much into the plug color...

I will start over again as you suggest. Don't get me wrong, the bike runs well - pulls hard - just stumbles immediately off idle.

[hikermikem]

The stumbles are gone!!

OK, now I'm answering my own post...

I backed off on the mixture screws & all is right with the universe. No more stumble off idle. I can blip the throttle as fast & hard as possible and the engine revs right up !!

I guess I was too hung up on plug color at idle. This bike is now officially "mean".

Thanks all,

Mike