compression testing a 2 stroke

[cedric]

I believe you correct DB, you are always supposed to hold throttle fully open when compression testing.

[crazy2wheeler]

Yep. Wide open.

Have you used that tester on other bikes? If so, what numbers were you getting on those other bikes?

(compression testers can be not all that accurate, but they are useful when comparing multiple readings with the same tester).

If you did use that tester on previous bikes which read 155-160, then this new machine was finally built right by KTM! (the 300's seem to always have more room [bad thing] in the squish than the 250's).

And, as far as high comp #'s go, my '05 300 put out just over 200 psi after the top end work, and it never needed race gas (at sea level it may have, but the lowest elevation I ever rode it at was Kook). I have always added a little octane boost in with the shell gold fuel though.

D.

[dirtyboy]

I think my compression tester is wacko. I'm gonna guess that it is reading way too high. I threw on the SX head that I usually run and it read 210 psi. At least it shows that the SX head is 10 psi higher than the XC head.

Back in '07 when I first got my SX head it read 165 psi stock and 195 with SX head. So I think that the new XC head has a lot more compression stock so the SX head may not be necessary.

[Knuck]

250 should be between 165 and 195 depending on model and modifications. I think the 300 is very similar.

I have mine tuned up for 195 when new and when it drops below 190 I replace the rings. Second time gets Piston, rings, wrist pin and bearing. But maybe I'm over cautious.

Maybe not. I got lazy and let mine go a little too long a bam! Shattered piston skirt. I got lucky and was able to get a hone on the cylinder but it was still over $1000 in parts.

[crazy2wheeler]

250 should be between 165 and 195 depending on model and modifications. I think the 300 is very similar.

I have mine tuned up for 195 when new and when it drops below 190 I replace the rings. Second time gets Piston, rings, wrist pin and bearing. But maybe I'm over cautious.

Maybe not. I got lazy and let mine go a little too long a bam! Shattered piston skirt. I got lucky and was able to get a hone on the cylinder but it was still over $1000 in parts.

The general rule of thumb I've heard is when your compression is down by 10-15% over its peak compression (newly run in piston/rings)... so really, for a bike that was giving 160psi at best, less 10% - 140 to 145 psi would be the lower limit.

D.

[kawasakijockey]

No not a motocross guy. Only trails...just bought the wrong bike haha started with a kx250 and now the sx250. One of these days I'll buy a "bush" bike. But for now i just wanna take care of my baby so she takes care of me. Seems I'll be real busy this winter better put the heat on in the garage...

[Dobi]

No not a motocross guy. Only trails...just bought the wrong bike haha started with a kx250 and now the sx250. One of these days I'll buy a "bush" bike. But for now i just wanna take care of my baby so she takes care of me. Seems I'll be real busy this winter better put the heat on in the garage...

Don't discount the 03 250SX. I raced that bike for 2 seasons as a bush bike and loved it. it needs suspension revalving and a flywheel weight. Once that is done, that thing is a rocket.