How do you do a Compression test?

[TripleR400]

how do you do a compression test? what all do you need?

[400eXr1d3rZ]

You need a compression tester, you take the spark plug out and screw/push the compression tester into the spark plug hole, and then you just turn over the motor.

[GPracer2500]

You'll need a 12mm plug hole adapter--some kits don't include this, some do. And remember to hold the throttle wide open when you're cranking it over. Keep cranking until the gauge stops climbing, which should be pretty quick on a 400EX.

Be aware that if your cam utilizes an auto-decompression mechanism (the stock cam does, most aftermarket cams don't), the figure will be skewed downwards. The number will still mean something, but it may seem unexpectedly low. Just be aware of that.

[TripleR400]

thanks for the info... where about could i get a compression tester?

My 400 just feels like its lost some power. i dont know if its time to rebuild it or just the summer heat messing with me.:confused:

[GPracer2500]

Last I checked Walmart sells a comp tester for not too much money. It includes a 12mm adapter. I've got one. It's not the best quality but I checked it against two other gauges and they all read the same.

As far as feeling like you lost power: Your jetting may need to be corrected for the heat. And even when the jetting IS right, there will still be a little less power in high temperatures.

Compression tests can reveal problems but discerning small compression losses can be tough unless you've already got a baseline for what your engine pumped at some previous time. For example, lets say it pumps 150psi (I'm just making that up). Well, did it use to pump 160psi or has it always pumped 150psi. See what I'm getting at?

This is why it's always a good practice to take a comp reading when a bike is new or after a rebuild (after a brake-in period). Note the air temperature too. Comp readings will go up when it's cold and down when it's hot (not gonna be a huge difference but if your trying to discern small changes that can matter).

[TripleR400]

thank you that makes total sense man... Do you have any idea what a new stock 400 psi reading would be... i have a stock 05

[GPracer2500]

OEM service manual says 100 - 128psi.

I forgot to mention that typically you always do a comp test after warming up the engine.

[TripleR400]

thanks alot for the info... possibly gonna rebuild it this winter and put some goodies on it...