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· Registered
414 Posts
Discussion Starter · #1 ·
While checking the valve clearance recently, I also checked the compression with two compression testers. The first had the rubber gasket fitting and the second screwed into the spark plug hole but only about 1/2 as far as the spark plug.

The r450 was 120 psi. I'm guessing that it should be at least 140-150 for a newer stock r450. Notably, the spark plug was bright white (looked like a new plug).

Likewise, I installed a Tudors 450 kit on my z400 w/ a claimed 12:1 compression ratio, and right after the installation it only read 110 on the compression test. This was with Hotcams with auto decomp.

I'm not sure if I have a problem on my quads or if the auto decompression cam causes the compression test to give a false reading. If so, how can you check the compression?

· Moderator
1,381 Posts
I guess you could probably figure out a way to hold the decompression in. With tape or something maybe?

I wouldn't necessarily call the decompression tests "false"... they will just be lower than normal. IMO, as long as you have a baseline for when the motor is new and broken in, then you can watch your compression as it lowers and the motor gets tired. You can still monitor your engine condition the same, just on a different scale. Kind of like dyno #'s..... not a reason to compare dyno to dyno, it is a just a tool for your own tuning. #'s are just #'s... blah, blah
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