Topic: Off-idle stumbling on throttle

[emlupi]

Good morning all,

I have a 750K2 which runs very well except for a slight hesitation/stumble when opening the throttle from idle. I have done all the tuneup items (valves, ignition timing, etc) and also carb items (thoroughly cleaned all jets, passages, clear tube test, insulator clamps, vacuum synch, etc). Once warmed up, it idles very smoothly and consistently. It has good power all through the rev range and good roll-on throttle response. Just a good, solid running bike .

However, there is this very annoying hesitation and slight stumble on initial throttle opening. It seems as if the engine is going to stall for a brief moment and then it clears and runs strong from there.

I am going to assume that my issue lies in the pilot circuit somewhere. Will adjustment to the air screws help to resolve this tendency? I would like to hear other riders experiences and where to look for resolution. Thanks as always.

[PeWe]

I had similar with my Mikuni TMR32, opened fuel screws more for richer mixture.
Air screws on stock carbs.

[flybox1]

If there is a mixture issue, a proper idle plug chop will reveal offending carb(s), and hint at which direction to turn the IMS

[TwoTired]

The bike uses all stock components?  No mods?


The clue is in how the engine recovers from "stumble". 
If it burbles back to running smooth, the mixture was too rich to support proper combustion during the transition.
If it suddenly resumes normal operation like it was just switched back on, then it was too lean to support combustion during the "stumble".

If a stock arrangement, and determined too lean, turn in the air screws or apply a bit of choke as a test proceedure, for idle mix enrichment.
If too rich, lean the idle mixture screws.

Cheers,

[emlupi]

Thanks, there is a little bubbling during the stumble and then it clears up and begins to run stronger and smoother. I will do some adjusting on the air screws and see if that clears it out. BTW, the bike is completely stock.