Motorcycle Repair: Bike speed problem, clutch springs, friction plates

Question
Hi,

I have Yamaha R15 V2 which is 150cc, 17Hp machine.
Issue: Bike doesn't go above speed 80-90Kms @ 7000-8000RPM. Which used to go earlier when it was new.

Recently I got 5th service done for it. However before servicing there was chain slack problem. And I informed it to mechanic. As per service guy he fixed the slack. But the above issue is still existing. To reach 80Kms above I need to rev engine above 9000RPM. And it is not a normal thing as I know the bike used to clock 100Kms @ 8.5K - 9K RPM.  What could be the issue here? Please advice!

Thanks,
Sachin

Answer
Hi Sachin,

If you have not done it already I would have the engine
checked for compression and compared to new specs.
Ideally cylinder pressures above 150 psi are best.

Next is the clutch as the friction plates may be slipping
at higher speed and loads. Some new plates and possibly
clutch springs may help here.

Also, you should do a general tune up including
air filter, tire pressure, spark plug, carb cleaning and valve clearance adjusted.
Check the exhaust pipe for any restrictions that might lower power.

The chain should not be overtightened. When you turn the rear wheel
there will always be a tighter spot. Also when you put weight
on the bike the chain may get tighter. Keep an inch of free play
at the tightest spot of the chain.

If the bike is in good tune then I suspect the clutch is slipping
and plates will need to be checked or replaced.
This often happens in higher gears and the clutch may feel okay
in lower gears.  The higher gears put more strain on the clutch
when you open the throttle and then you need more engine RPM
to compensate for the slippage.

WS
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