Stupid power valves.

[Lozza]

Part number 11 is driven off??? Is that some sort of a clutch arrangement(Nos 14-21)?
Got a lot going for it o ring head, good port layout......... 

[Nathan S]

11 is driven by the primary drive gear, or possibly via the water pump gear.
14~21 are just the governer bits. Presumably the spring tension can be altered to change PV operation relative to revs.

Parts pics are of 82 YZ250J. It's a pretty generic set-up for YZs of that era though.

[DJRacing]

Part #11 is driven off the main gear on the crankshaft. Parts 14-21 as the motor's revs build up the ball bearings are forced out which makes pt #19 compress the spring. The actuator arm which sits in pt #17 is moved back and forth which in turn moves the powervalve open and closed. Its a very simple thing but it does require maintenance.(a good quality oil helps alot with less carbon build up). The spring needs to be replaced to stop the p/v from fluttering. I think alot of people replace the o-rings with standard ones when they should be viton (heat proof). Plus the p/v its self isnt cast very well and sometimes needs a bit of file work to make sure it moves freely. The bushes at either end can wear and become out of line which causes them to jamb up. Just remember the heat they are under and every time ya twist the throttle the p/v gets a work out so all the parts need to be in excellent condition because when they fail they cause big problems. But they do work and ya can fine tune them to open up earlier or latter depending on how you like to ride and what type of tracks you ride.

[DJRacing]

  ya got in there first with the answer Nathan.

[BONEHEAD]

GUYS , this is just simple maintance , it should be checked after installation on new engine and after run in period , this should also be checked for adjustment and operation before the next meeting, it should have no sticking points when disconnected from its actuator and should be free off carbon, i have cured many performance concerns due to this PWR valve being unserviced and gummed up , its also very important that the correct machining is carried out when the cylinder is  rebored , yes it may be a pain in the ass to service but this will hopefully save the agravation it will cause. simple.clean it  adjust it and preserve it.

[Lozza]

As I suspected a shit driver mechanism. Look for a RZ servo on ebay,servo conversion is the stuff of your dreams