Compression testing (YZ250)

[jp2396]

Since I've read my manual (couple of times), I cannot find anywhere where it says what the cylinder compression should be ie: 185 to 210 psi as an acceptable range. Does anyone have a formula that can be used to figure a relatively close baseline reading? I'm at about 4700' in altitude and tried a test once and came up with 165 psi with 2 hrs. on the motor. Any help is appreciated.

 

[Studboy]

JP,

I get 235 PSI with a fresh topend stock compression on my 02 YZ250 (basically the same motor), and I'm at 4200 feet.

Are you holding the throttle wide open and kicking it until it won't go any higher?

 

[jp2396]

Studboy,
I am holding the throttle wide open. The highest I can get it to register is 165. Maybe it has something to do with the "Harbor Freight" compression tester, and before anyone jumps me, I know I should have bought a reputable brand. The MAC Tools tester doesn't look to bad for the price. Could the lower reading be that the engine hasn't been run enough? I ran it like the book said for break-in and it definitely has plenty of power.

 

[KX02]

It could just be your comp tester. I got mine at auto zone and I could only get a reading of 165 on a fresh top end on my bike which should be 190-200, but I did have to use an andapter and I know that will lower the reading.

 

[Studboy]

KX02, you are talking about a 125 though.

The comp. tester I have is an autozone, but it was fairly expensive and I think that it is really accurate from different machines that I have tested.

I have heard that the harbor freight testers aren't very accurate, but I haven't seen one first hand.

 

[Studboy]

Oh... and my bike makes about 228-230 before the rings are seated... right after breakin I pick up an additional 5 psi or so.

 

[jp2396]

Studboy,
So what do ya think, pick up a different tester and give that a shot? I have had hit & miss luck with their tools. I bought a chassis punch set and the second knockout I did broke the bolt, and this thing was supposed to stand up to 16 ga. steel. You get what you pay for, the tester cost me $20, so I'm not out a bunch on money.

 

[Studboy]

JP, I would definately spring for a quality tester. Then you know what you really have going on, plus you will not second guess yourself in the future because of the tester.

I would think that if you are only getting 165 still that you have a problem somewhere.

 

[jp2396]

Thanks for the info on this one. I'll pick up a quality tester and go around with it again.