The standard clutch have sometime the rubber silent-blocks inside the clutch gear lon 140 to 160 and inside the primary gear on 88 and 125.
We seen this trouble also for some Takegawa and Daytona engine
With these rubber, no more trouble
Video of our reliability tests:
These stronger silent-block are suitable for engines over 20hp
 
Replace these rubber can be made with a drilling machine but it's better to use a milling machine with a tray divider.
Milling 4 rivets holding the plate to back up far enough to place the head of screw 90°
Made a thread for M5 screws countersunk head 90°
Milling the last 2 rivets
Do the thread for the last 2 holes