4K WOT Stumble

[Coffinator]

Hello!

I've been running a stage 7 setup with vboost eliminated for some time. WOT just fine. I installed the vboost assembly recently, without making any other changes, and now have a bad stumble at 4K WOT. Should i drop or raise the needles?

 

[maleko89]

Hey, I'd recommend doing the needle test.

1) 4th gear, 4K RPMS: WOT until 6K RPMS; let off throttle and if it speeds up, running rich and lower the needle.

2) 4th gear, 4K RPMS: WOT thru 6K RPMS; at 6K, give it some choke; If it speeds up, running lean and raise the needle.

If neither condition happens, needle position is fine.

 

[Coffinator]

Hey, I'd recommend doing the needle test.

1) 4th gear, 4K RPMS: WOT until 6K RPMS; let off throttle and if it speeds up, running rich and lower the needle.

2) 4th gear, 4K RPMS: WOT thru 6K RPMS; at 6K, give it some choke; If it speeds up, running lean and raise the needle.

If neither condition happens, needle position is fine.

Thanks, I'll try that! Also, after installing the vboost assembly, I was able to sync the carbs. It looks like one of the cylinders needs more throttle to run at idle than the other three, making my engine run at three cylinders when they are synced, and on four when the one carb gets more throttle. Seen anything like that before?

 

[Coffinator]

With vboost open, the bike was running fine with the clip in the third groove plus a shim. With the vboost reactivated, I had to move the clip to the second groove from the blunt end.

 

[redneksoldier]

Thanks, I'll try that! Also, after installing the vboost assembly, I was able to sync the carbs. It looks like one of the cylinders needs more throttle to run at idle than the other three, making my engine run at three cylinders when they are synced, and on four when the one carb gets more throttle. Seen anything like that before?

Sounds like a plugged idle circuit to me. You won't notice it without the vboost installed because the crossovers will allow that cylinder to pull fuel/air mixture from the paired carb even at idle. Once you installed the vboost, the crossover is blocked by the butterfly and does not allow the fuel/air mixture through which basically kills the cylinder until you give it some throttle to allow the plugged carb to start fueling.