I could really use some help. My '91 Softail Custom is breaking up around 4500 rpm. I have replaced everything that could be causing this that I could imagine. New Twin Tec ignition, Accel Super Coil, Plug wires and plugs. Also new ignition sensor and new Mikuni HSR 42 Carb and manifold. Bike runs better than ever up to 4500 rpm then stumbles. It never did this before and has run perfectly since new. Mods are Screamin' Eagle heads, Flat top pistons, and Andrews EV3 Cam with baffled 2" drag pipes. Any ideas would be really helpful. I was going to bypass the ignition switch by jumping from the + terminal on the coil to the + terminal on the battery and see if it eliminates the stumble. Maybe a loose wire or something.
Thanks
Buzz

 

When was the last time you changed the screen on the petcock and cleaned your tank? Sounds like fuel starvation

 

I put A Pingel petcock on there probably 25 years ago. Never been off. I'll check that.
Thanks

 

So it was running good then you did the engine mods, heads, pipes and Mikuni? And now it's not running above 4500 rpm? 2" drag pipes aren't helping your situation.. What main jet are you running..? EV3, now there's a cam you don't hear much about any more...great cam in it's day..

 

No all of the mods were done years ago, Heads, pipes and Mikuni HS40. Just recently swapped out the Crane HI4e ignition for the Twin Tec and the old Mikuni HS40 for the HSR42. Wanted to upgrade the HS40 years ago for the newer 42 just never did it. Trying to fix this problem I thought I would just freshen some of these 20+ year old parts.

 

try a 190 jet in the main...

 

This is a shot in the dark as I don't know **** about twin tec modules but what rev limit do you have it set to?
I ask because when I got my motor S&S their super stock ignition was set to 4.5k initially for engine break in.

Is there any chance your rev limiter is set too low?

 

Forgot about that one... Good catch Mike...

 

Thanks guys. I do have it set for 6k rpm. I am going to pull the petcock today and I think that may actually be the problem. I have never pulled the petcock and I bet it will be really dirty. It also only stumbles under load at 4500 rpm as I blipped the throttle in neutral beyond 4500 rpm briefly with no breakup. It really does sound like fuel starvation the more I think about it. If that doesn't do it then it will have to be a faulty wire somewhere that is causing intermittent high rpm issues. I'll try running a 16 gauge from the + side of the coil to the + side of the battery to bypass the ignition switch etc. and see if that does it. If not I'll have to check the additional wire added for the single fire ignition to the coil and the other wires in the harness between the Twin-Tec and coil. I'll let you know how it goes and thanks for the help. Have a great Memorial Day weekend.

 

Well drained the tank and checked the petcock and all OK. Problem still there but I hooked a wire from the coil to the battery and problem gone.... so now I need to isolate what the problem might be. Any suggestion what to check first? Ignition switch?