Power Valve wasted? YZ250 89 Pics inside.


Hello everyone! I'm new here (new to MX and also to this forum) so just a quick greeting from England, London (Kent)! :thumbsup:

Ok, so i've taken on what some would consider a handful for my first project bike... an 1990 YZ250 (1989 Engine) Im part way stripping down my bike and i've come across (what could be) my first problem, a chipped power valve?

See pictures below.



now.. i assume this is some kind of stone chip and is not a good? Weirdly the bike was running prior to the rebuild so im unsure what to think. I assume performance would have been effected? The head is showing no signs of damage nor is there any damage to the piston! What do you guys suspose happened?

Everything seemed to be going so well until today :lol:

Are there any kind folk have a spare power valve doing nothing? I get the feeling this part will be hard to find as it's almost 22yrs old! :)

Bike so far.. (head has been removed since this picture was taken)


Any help appricated peeps!

The power valve is ok.

It has chipped from the side that is facing away from the piston.

Just check that there aren't any sharp edges (where it broke) that could seze the valve. Also clean the valve and check its smooth movement in cylinder.

My preliminary thought is that someone wedge something in the exhaust to prevent the valve from spinning while taking the bolts loose to remove it. right wrong or indifferent I always use a wooden hammer handle for that but I can see someone trying to jab a screw driver or something to do that.

I would use it if moves freely in it's bore and doesn't interfere with the piston when it the close position.

Mat & Jsantapau - thank you both for your replies.. I've got no choice but to re-use the part in anycase as i cant find another anywhere. I feel less apprehensive now you both have confirmed what my dealer already told me today. If the perfromance impact is minimal then i'll carry on using... after all it is 20yrs.

Jsantapau - i have no doubt you are correct... i too used a woodern hammer handle to wedge the valve stationary. The valve spins clear of the piston and rotates in its bore without issue.

Thanks again for your help (both of you)


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Mikeybabbel
      Hey guys, I just picked up a running 03 yz125 a few weeks ago and upon tearing down the top end, I noticed the cylinder head looks a little beat up. Is it possible to just clean it up or am I better off just buying a new one? Thanks guys. 

    • By faizakhtar125
      Hey guys I just bought a bike last night. So the seller said the bike will turn on but then die. We didn't try it because it was really late at night and it was raining (seat was wet). But he said if you bump start it, it will turn on and ride. So what do you think the problem is? I just want to make sure it runs... I probably won't be able to bump start it till Saturday (rain)... It's a four stroke, and I know absolutely nothing about them. Thanks.
    • By neonmarine
      Hey guys, 
      I've used this forum as a "guest" for awhile now and I thought I'd share my build now that it's all coming together.  The primary purpose of this project is to help me train for roadracing my 2009 R1 at lower speeds as well as giving me a cheap alternative to racing the big bike. 
      Here's what she looked like when I picked it up from a local shop.  Paid $800 cash for the bike as you see it with 17' rims, FMF pipe, EBC 250mm front rotor and clip on handle bars.

      Fits PERFECT!

      Shake down run at the Herrin Compound last summer:

      As she sits today:

    • By 125 yamaha guy
      i bought my yz125 from a dealer and they said that the clutch was broken so i bought it and it worked fine it would run move and everything and then it started to act up and now only first gear works barely though. when you put it in first it acts like its bogging down but its not and when you go into gears 2-5 it wont move i dont know whats wrong with it im assuming the clutch is slipping or something if you know please leave me a comment