Compression Test

How do you do a compression test, and how do you read the results? What measure are the results in?

Thanks

[oynot400]

A compression tester goes into the spark plug hole. Hit the starter button and let it turn over a couple of times. There is a needle gauge at the end which will read in PSI. I am not sure what the reading should be, it all depends on the piston that you have.
Hope this helps... :)

[fourtrax400ex]

i take the compression on a brand new 400ex a couple of week ago and it give me 90psi on the tester, i think it supposed to be higher than that but maybe the automatic compression release on the cam is the problem?!? is there a way to have te compression without the automatic release?? i check also my 1999 400ex after that and they only indicate 70psi but since last weekend i have now a 416ex:D

make sure when you take your test to have the gas off and the throttle pinned

make sure when you take your test to have the gas off and the throttle pinned

Why do should you do this?:)

[booneTRX]

On a 2-stroke you need to hold the throttle pinned because of the piston porting. A 4-stroke does not need to be done this way.

[Chef]

is there a compression release on aftermarket cams?

[oynot400]

Originally posted by HOTT440EX
is there a compression release on aftermarket cams?
The compression release is not part of the cam. It is more part of the valve assembly. It all depends if the release was removed when the cam was installed. I think that the more agressive cams require to have it removed, but I am not 100% sure of that.

[shockwave67]

You absolutely do have to open the throttle to do a CORRECT compression test on two or four strokes otherwise the motor will not turn over enough to pump up. With the throttle closed the engine cant intake air and create a good pumping condition. Compression requires AIR being compressed. I also reccomend using a battery charger on a built motor to insure adequate cranking revs while checking. Crank for about five seconds until reading maxes. You will see a 50-75 psi increase when you open the throttle vs not opening it. If you had 90 closed you are ok. It will be around 150-160 with the throttle opened.Mine reads 150 closed and 245 open throttle. 13/1 piston. Stock motors on a 400ex will read from 125-175psi.

[out4sand]

when I had the motor built in our 400ex they tested the compression said the same thing that shockwave67 said, the throttle must be wide open to get a true reading. Before the rings and such were seated it tested at 207.