Hi Thomas,
If your mixture is too rich you can often tell as the engine doesn’t run free and and happy. If when you make an upward gear change, when you reopen the throttle does the engine fire cleanly as it should or fluff and four stroke for a while suggesting an over rich mixture. If you ride say 50 mph, you would be barely on the needle. Try dropping the needle one notch to see if it makes a difference.Has it got the correct carb slide fitted? Have you got enough ignition advance? They can all come into it. It is not likely to be the main jet. If the engine ever four strokes while riding on a flat road, it is too rich.
Regards,
Bob Mather.
