Motorcycle Repair: YAMAHA SR500

Question
My 1980 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
Hello Kevin,

Sorry for the delay, race weekend and away from home...

When you say "all fuel possibilities", have you checked the air cleaner system?
Air intake duct excluded, the airbox should be air-tight.
Goes without saying that the intake duct be present and not modified, otherwise the main jet must be replaced with a larger one.

The duct has three openings/air-passages. Close one of them with duct tape and see if the problem gets better.
If it does, either the main jet is clogged/too small, or, the air-box "sucks" more air than it should, probably through the filter cover.

Have you checked ignition timing? You need a timing lamp to do that.
Get back to me if you have one and need instructions on this.

Close the spark plug gap to 0.5mm and check engine running.
Check valve clearance (Intake 0.10mm, Exhaust 0.15mm).
Check cylinder compression pressure.

Let me know what the results are.

Best Regards,
Lazaros.