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Motorcycle Repair/Four stroke miss


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?

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

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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