How to compression test with the decomp cam?

I was wondering if the WR450 guys have anything in the manual that talks about testing compression?

I put the '03 YZ cam in eons ago, and am very happy with it. I have recently experienced starting problems, so I went in and found an intake valve out of specs. Waiting on the correct shim, but if that does not solve my starting problem, I am thinking my compression may be down. I have approx. 10,000 miles on my '98, so a freshening up of the top end is probably due.

Any thoughts about how to test compression with the auto-decompression cam?


My compression tester has a valve in it that holds the highest reading till you release the pressure through a push of a button.

The 03 exhaust cam decomp lever or "weight" would have to be blocked in a way that would simulate the engine spinning faster than what it would while kickstarting. No easy way of doing it ---looks like the valve cover would have to be removed then jam the decomp lever on the cam put it back together do compression check then remove the jam on the decomp lever. No easy way I can think of -also nothing in my 03 WR450 book about compresion checking not even psi range for compression!

Try kicking it over with compression gauge installed it (decomp lever) may release while kickstarting. There has to be enough compression to start the engine if the lever stayed open I don't think there would be enough compression to start engine.

By the way what compression gauge are you using its hard to find one with the small thread and diameter this bike has?

Yup, the auto compressor would have to be disabled somehow to perform acompression test. :D

I got my 10mm spark plug adapter from Snap-on years ago. :)

I haven't checked it out but you may not be able to disable the auto decompressor.If it can be done you could do your compression test with the valve cover off. Just make sure no foreign particles or dirt fall into the valve train while turning it over! :D

Otherwise we're only able to check compression with a leak down tester $$$$$$-they cost about 300 bones from motion pro. :D

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