My buddies Bayou is intermittently not revving up properly. It sounds like it is hitting a rev limiter. I do not think it is carb related. It sounds very much like other quads I have worked on with bad regulator/ rectifiers, which is what I think it is. The battery is holding at 13.3VDC though even when it is acting up. Stator output is around 36VAC across all three wire and resistance is about .5-.6 across all yellow wires. Could the VR be bad but not be putting out an enormous amount of voltage causing the limiter to kick in?

 

I would clean the carb out anyway, to eliminate it. If the regulator is kicking out 13.3v it is a bit low, but should not affect the running of the bike. Though a high resistance between regulator and battery (bad connections or earths) can drop the voltage, and may affect the sparking. Check them, and try a new plug, but it looks like a CDI problem.