400ex compression

[EXriderO2]

my compression on my 400ex is at 90 psi is that good or bad? if bad what should it be at. its a stock motor with just the intake and exhaust mods

[underpowered]

how exactly did you take the reading?

on a 400ex it is best to do it ona warm motor. Hold the throttle Wide open while cranking till the gauge stops climbing. with a stock cam, it is hard to get a good reading due to the decomp mechanism.

but anyway, 90 is low. even with the decompression mechanism it should be about 120 or so.

[GPracer2500]

^^^What he said. Make sure you hold the throttle wide open or the reading can/will come out low.

[EXriderO2]

i did it cold while the throttle was wide open, i was told i need vavle seals and piston n rings. will if hurt nething if i ride it the way it is for a few weeks?

[GPracer2500]

The risk is that if you've got lots of blow-by (and surely you do if you're only pumping 90psi) then the hot combustion gasses escaping into the crankcase can ruin the engine oil in a hurry. Also, you've gotta be burning oil.

If you keep riding it make sure you keep enough fresh oil in it. If it gets low or breaks down then other parts can be damaged ($$$$).

The above assumes that the reason you've lost compression is because your rings/bore are simply worn out. If physical damage to your piston is causing the comp loss then continuing to ride it would not be smart. It might fail catostrophically and ruin the engire engine.

[EXriderO2]

yeah it burned a quart in about 6 hours of riding

[GPracer2500]

If I were you I'd ride it as little as I could stand until it's fixed. If the oil consuption accelerates all of a sudden then you might run low without realizing it. And there's really nothing you can do about the blow-by contaminating the oil that is in there.