Motorcycle Repair: Four stroke miss

Question
My Yamaha SR500 single cylinder 4 stroke motorcycle runs fine until it reaches about 4100 rev's then it starts to run poorly and cut out. I've checked all fuel possibilities.  I've swapped out the coil and the cdi box but the bike
still stumbles around 4100 RPM..under a load. The Stator was found to be within tolerance per the book...reads. While in
neutral, reving the motor it smoothly revs to and past
4100 RPM. Any Ideas?

Answer
Kevin, if the bike has been modified with a different exhaust or intake system, it may need to be re-jetted richer.

Did you check the spark plug cap for excessive resistance? Most bikes use a 5k ohm resistor type of plug cap.

Make sure that the petcock and filters are all flowing well and that the gas cap venting is working.

I assume that the CDI box controls spark timing. Check it with a timing light when the engine is running to determine that the spark advance is functioning properly.

The pulse coil is part of the stator I think, so be sure that it isn't gone out of spec or wiring connections are not defective.

These bikes vibrate a LOT, so any kind of vibration-related issues can arise, including bad grounds.

Bill Silver