yamaha powervalve stop (i finally did it)

[yz250roost]

hey guys, just a quick summary of what happens with the 98 and earlier yamaha power valves, the stop on the left side of the bike is behind the powervalve cover, it is the black circle with the screws on either side, if you pull that off you will see on the inside there are two metal tabs that stop the powervalve from rotating. after a while the constant pounding on the tabs creates and indent and that can cause the powervalve to hit the piston, the stops on mine were worn down about an 1/8th of an inch and the piston had signs of contact with the valve. So I'm going to replace the powervalve and the stop plate itself, I urge anyone with these bikes to check this out immediately. post if you have any questions on how to do this, it was pretty simple and I'll get back to you on pricing.
Jeff

 

[Rcannon]

Jeff, thanks for posting this. I tried to ask the same thing over on Eric's forum. The message was dumped over here and forgotten. Thank You!

I pulled my 97 apart last weekend and know the parts your talking about. Mine have some indentation, but I wonder how much is too much. Eric, in his book, said to replace the stop when it is worn .30. I did not understand .30 of what?

Maybe when you get your new parts you could measure them. Then I believe I could be smart enough to subtract .30!!!!! Yes, me that dumb.

Could you hear any unusual noise when the valve was hitting the piston? Why not just grind the powervalve itself? This I do not understand! Thanks again!

 

[jmics19067]

I have Eric's first book which only goes up to 96 bikes. He says to replace them when they are worn .030" or .7mm . I visualize a straight line between the not worn area of the tab of the power valve and the tab on the stop and guestimating by eyeball the worn area to the thickness of a spark plug gap<.024>. Definately not a recommence way of measuring but if the wear between the two pieces starts looking close to my spark plug gap I toss them both in a strong metal container that gets picked up twice a week. Ona 94 yz 250 the power valve is close to 100$ and the stop plate is about 30$ I believe.
I guess the best way to really see if the tabs are worn to a critical point is to pull off the top end lay a straight edge down the exhaust side of the cylinder bore. Then see what the gap is between the leading edge of the exhaust valve to the straightedge and make sure there is enough clearance . But what that is I wouldn't know .

 

[yz250roost]

I just ordered the new stop plate and the new powervalve and the gasket for the powervalve cover, it ran me 140.00 for everything. If it were just the stop plate that was wearing down that would be a problem, but the problem multiplies because the stop tab that is on the actual powervalve itself is also being worn down, so basically take the measurement from the stop tab and then take the measurement from the other tab and add them together to get the total wear figure,
hope this helps
Jeff

 

[Rcannon]

Thanks guys. This is becomming more clear. I hope your scrap heap has more room. Mine is not long for the world.

Question.....Will it hit the piston on the "open" or "close"?????

 

[yz250roost]

mine seemed to be worn down where it would be close (exhaust port open fully), But the more I look at it, it doesn't seem like the area that looks worn would be able to hit the piston in any way, oh well, the stops on mine are definetly worn down and I'm sure keeping it fresh isn't a bad idea. take care guys