I just installed a mikuni 42mm bps setup on my gtr35 with 260 hours, at the same time a rapid r exhaust was installed, it is a 2013 cdi motor. The problem I have is off idle stumble when throttle is given. The idle is great the top end seems to be fine but if throttle is given to quickly it will not just stumble but will just die. I moved the metering needle up as high as it would go and it helped but did not completely solve the problem. If I cover part of the carb with my hand it solves the problem. The accelerator pump is a 70 nozzle. is there another adjustment for this that I am missing or is there a trick to solve this problem.


Oh yeah valves were just set at .005 intake and exhaust, new auto lite platinum spark plugs, neither help the problem at all.

Any help would be greatly appreciated.

Have you adjusted the accelerator pump? Make the paddle touch the rod, then back off a tad. Also make sure the nozzle is spraying fuel on the needle.

Also put the jet needle clip back in in the middle. That is used for acceleration and mid range throttle past 1/4 throttle. Adjust it as needed to get a good AFR/ response from the engine.

I ah e played with the adjustment for the accelerator pump and and tried opening it up early tried opening up later opening early helped slightly and that’s where we had it set. But it continues to stumble the needle adjust was moved due to Mikuni instructions for off idle to 1/4 lean condition. I saw that the Harley guys were changing the air pilot jet in this situation from a 1:1 to a .7 or.8:1 although I am not sure of the effects in this situation.
I am leaning towards going back with a stock carb, it seemed to perform as good on the top end and better on the bottom.
If so I will have a Mikuni kit for sale with less than an hour on it

If your motor is stock it's not the best candidate for a mikuni imo

Mikuni with a stock motor and stock head ports/ valves is not a good combo. Just not enough port velocity for that large of carb

No way I would ever put the POS stock carb back on. Have the heads worked and keep the carb. You will be happy you did.

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