Compression test

[Fear-Nuttin]

I'm preparing to do a compression test on the 400 Scrambler (2 stroke). What is the proper way to do the test, and what should the gage read for good compression?

[†2005 400ex†]

check the manual for the stock compression, i usually just start the engine at BDC and rotate the engine to TDC

[†2005 400ex†]

you dont have to be precise with the piston positioning though, you can just crank the engine over a couple of times

[250X_project]

Yeah check your manual. The rule of thumb for a small 4-stroke engine is good compression runs around 130-150 PSI, 100 is usually rebuild time. (this is only a rule of thumb and there are exceptions!) I don't know if this would match for a 2-stoker though...

tighten in the tester to the plug hole, then open the throttle and crank until reading no longer climbs. Crank at least 4 times. Best results on an engine at normal running temp.

If you don't have a manual make a post in the Polaris section to find the specs, if you haven't done so already.

[250X_project]

And running the test a few times, and/or using more than one tester helps accuracy. If you do this exsessively for some reason keep an eye on the oil, it will get black faster with the engine cranking, but not firing.

[Fear-Nuttin]

Thanks guys....major help!

[250X_project]

Glad to help. Let us know how it turns out.

[DieselBoy]

Screw the compression tester fitting into the hole.
Turn over the engine, with the killswitch engaged.
Read the compression tester and see what compression it peaks at. that gives you a good idea of how well the rings are sealing.


*Always ground the spark plug otherwise an electrical failure may occur.

*Note a cold engine will always have a lower compression reading.