New top end - compression?

T

TLJ

Member

Sep 28, 2004

5

0

• Oct 8, 2004

• #1

Had YZ 125 bored/honed once over. Everything measures good. I am getting 100-105 PSI on compression check. Is this exceptable for new top end? I also did a leak down test and am losing air around the power valve shaft (push rod side). Manual shows no seal or o-ring for this. Any suggestions?

 

R

reelrazor

Member

Jun 22, 2004

340

0

• Oct 8, 2004

• #2

That compression is low. If it hasn't been run yet, it may be acceptable. Break it in and check it, or maybe try a different/better guage.
You did hold the throttle wide open when you tested it didn't you?

Yes, leakage at the powervalve is common. It is downstream of the piston so it doesnt really constitute an air leak.

 

T

TLJ

Member

Sep 28, 2004

5

0

• Oct 8, 2004

• Thread starter
• #3

I did hold the throttle wide open with the kill switch on. I will try another gauge first before breaking it in. What should the compression be with new top end? Last time I had the top end done it had 130 PSI. I am hearing that I should have about 180 PSI.

 

R

reelrazor

Member

Jun 22, 2004

340

0

• Oct 8, 2004

• #4

Try another guage if you want to before running it. Unbroken-in compression testing really doesn't mean much. a) the rings haven't seated at all and will not hold what they will after even a short running time. b) you haven't any heat in the moving parts, which affects ring sealing as well.

In short, if you think the refresh went okay, start it up and run it.