You ask we answer. Here's the latest in our "Ask a Mechanic" series.
Q: Last time I went riding my engine sounded full throttle but I wasn't gaining speed. It sounded like I was hauling the mail but it was definitely not hooking up. Something wrong with the engine or did I burn the clutch?
A: If the engine sounds normal then yes you have a slipping clutch. This is quite frustrating especially when racing because you have the throttle pinned while you pop it up gears but you don't go any faster.
If the clutch slips a lot the bike revs up faster than normal but doesn't move so you would be forced to shift through the gears rather quickly. If the clutch was worn out entirely you couldn't move at all without feathering the throttle and clutch. The slipping clutch, even when slight, is pretty obvious. Smell the oil after a couple hard laps. It will smell burned with a slipping clutch.
Other situations similar to a slipping clutch include loss of compression but this causes the bike to hesitate, bog and have a lack of power. A stuck or sticky power valve on a 2-stroke causes issues where the bike seems to run fine but has a lack of power and will also not rev out correctly. Same with a fouled spark plug, or even incorrect jetting but all of these symptoms differ from the slipping clutch because they won't allow the engine to rev. In many cases the bike might run and ride it just lacks power.
Check the thickness of the clutch and pressure plates. Also, keep in mind the type of oil used. Oil not manufactured for a wet clutch will cause the clutch to slip even if the clutch and pressure plates remain in spec.