Compression test showing 20 PSI?

Hi all,

I was about to buy my first bike yesterday - a WR250X - and, while in the shop that thoroughly checked the bike before the purchase, discovered that the compression test showed 20 PSI.
They told me there might be a leak and that the valves might have to be tuned, but that it might also mean the engine needs rebuilding...

Either way, that all seemed weird, since the bike ran really well (great acceleration, easily hit top speed, everything seems to be working well).
I looked around on the net and it doesn't appear like it would be possible to even start the engine with 20 PSI.

Is this a possibility or did they screw up the test? I have very little technical knowledge and relied on them completely for the test and I'm wondering if they failed me or if there might really be an issue with the engine!

I'm sure I'll get flamed, but here goes ... A compression test on a single cylinder 4 stroke motorcycle is worse than useless. Most engines in this class use a compression release, and are supposed to have low compression at cranking speeds. Even worse, gages vary and testing methods vary, so without other cylinders to compare (the only value of the test) the test is more than worthless. Plus the things a compression test could find are either obvious, or better found with other methods.

Have the shop do a proper leakdown test with any compression release disabled for better info. I wouldn't use the mechanic that administered the test for motorcycle work anymore. Your test ride told you more than the compression test by far.

Whoever did that compression test is ignorant. Timing would be completely, and I mean completely off,causing the exhaust and/or intake valves to be open for compression to even hit 20psi, and it certainly wouldn't run.

Going back to the methods bit, did they test it with the throttle open or closed? I'm sure the service manual states the correct method and acceptable readings.

Anyone with a brain woulda thought "strong running engine... 20psi... that don't match up!". I certainly wouldn't let them work on anything of mine.

Section 3-9 of the service manual. ... Wide open throttle ... Valves adjusted ... Full Battery ... 105 to 135 psi. But gages vary wildly, and it is best administered as a comparative test, or as a go/no-go test. IMHO.

Yamaha provides no data that I can find on the crankshaft RPM that the compression release allows full compression. I hunted the service manual. The compression release is clearly shown in the engine section of the service manual.

I suspect that given a high enough cranking speed, you can get a "real" 100+ psi cranking reading on this engine. Full throttle and full battery should do it, but if I got a low reading on an otherwise good running engine, I'd investigate the test conditions long before I'd worry about the engine. Hell, on this engine I'd only use a compression test to look for engine mechanical problems related to a no-start or severe smoking or performance problem. Even then I bet I could do mich better with a leakdown or a fiberoptic camera. To use a compression test for a "wear evaluation" on a good running machine is just useless. it leads to so much misdiagnosis it is shocking.