B400 stumbles in 2nd gear?

[Monty_Greer]

91 B400 18,000 miles--stumbles hard in 2nd gear with moderate to full throttle from 5K to redline rpm.  Only happens in 2nd gear and hasn't always done this.  Bike is US spec with afermarket can and intake snorkel removed, new plugs and DynoJet stage on jet kit.

I'm stumped, I've bypassed the neutral/kickstand circuit but no improvement.  I can't tell if it's ignition or vacuume signal related, but it's so abrupt that I think it's ignition.  I have not had the igniter module tested... as it works fine in the other gears.

Any help or thoughts would be appreciated.

[tomacGTi]

Probably your jetting, more than likely too lean.

If you notice it under hard acceleration, in any gear it's likely the culprit. This is of course assuming that you have fresh wires, good fuel delivery (make sure the vacuum signal to the petcock is not kinked or run it in prime), charging system in check.

I would notice lean running if you accelerate hard and feel a bucking as you got onto the mains. It feels like a rev limiter but it's not. If it's not too violent, you might be 1-2 sizes too small but with an open airbox you're more off than you think.

With my setup I'm now running 120 Mikuni mains with my DJ kit. IIRC DJ's "big" jets that came with their own numbering were only 100s. With your setup, you may have to step up to a 115 Mikuni or put the snorkel back in and run what's in the carbs.

Welcome to the fun that is jetting, just wait till the weather gets cold and you have to do it all over again.

[z_poper]

Have you figured out what the problem was?? My b4 has been doin this for almost two seasons, I'm almost certain it's the shift fork. If it only does it in the one gear then it's a transmission problem.  I either shift past 2nd gear quickly or I skip it altogether - it's a pain in the ass but it's all I can do until I can replace the entire tranny.  I'm not a pro, this is just my opinion on the problem.