Currently working on a 1992 Yamaha Big Bear 350 that idles perfect but cuts out with any kind of throttle. We replaced carburetor and head to carburetor boot.

Put in a new spark plug, the old one was black as hell, unsure if it was caused by old carburetor or current misfire. This problem similar to the rev limiter you see on newer models when differential is locked or in reverse. I don't think this machine has that feature but I could be wrong? I suspect an ignition component is breaking down but at the moment I don't have access to a wiring diagram or workshop manual but I'm working on that. Are the e-manuals any good?

Any ideas?

 

Can't help much on fault diagnosis as you seem to be going through it logically. I find official Yam manuals are a bit awkward to use, as some of the e-versions are supplements for earlier books and without those, are incomplete. Plus, being for dealer mechanics, are brief on certain aspects. For example, I have the Grizzly 600 e version book, and had a leaky carb to fix last week. The float bowl assembly is the weirdest I have ever come across, to re-fit you have to put float and needle valve assembly into float bowl and lower carb body onto it, then turn over to fit the screws, does the book have anything to say about it, no. The books are better on electrics though.

 

Problem solved, replaced the ignition coil.