Compressoin test = 55psi?

[JAM]

I just bought a 400ex and had oil pouring out of the lower crankcase breather port so I did a compression test and got 55 psi. What should the compression be? Is it rebuild time?

[Wheelie]

Personally, I don't like compression testers. I use a leakdown tester. A compression tester will show cranking psi, a leakdown tester will tell you where the engine is losing that cranking psi.

[JDMThoughts]

Cylinder compression specs are 100psi-128psi. From my understanding with cars, if the compression is lower than 30psi that the spec it needs to be addressed, since it appears you have 1/2 the compression I would definitely be doing a rebuild.

[2004exrider]

The most accurate way to measure the psi is to get the engine up to operating temperature (if possible in your case), after that shut it off and hold the throttle wide open and let it turn over until it reaches its highest point on the tester. The way to tell if you need a new piston and rings is to poor a little oil in the cylinder and turn it over. If the psi jumps up then they need replaced. If not, it could be a valve problem.

Jimmy

[Dupontster]

That seems pretty low. On my 426 I think it had around 125-145. My 450 stock with HRC kit was 175. I would definetly tear that top end apart.

[groutman]

my stock 400 has 80psi when i checked the other day seems to run fine though

[Jared Coderre]

The stock quad has a decompression release to aid the bike in starting. When checking the compression this could be a factor. 55psi seems low for it.