How do you use a compression tester??

[dougsr.874]

I just bought a new compession tester at a local parts house but need some instructions....I searched the threads but can't find what I need to know...............What is reading for a good HT engine......some say 70psi, some say 100psi...can you simply elevate the rear wheel and spin it by hand to get a correct reading....or do you need to affix the gauge to the bike somehow and pedal it to get a good reading....All help appreciated....

 

[Fabian]

Firstly you disconnect the electrical input wires into the CDI so that when spinning the engine over you don't burn out the electronics.

Secondly you test engine compression with the throttle set at wide open.

The last part is to spin the engine over any way you can, which is usually by pedaling the bicycle.

Typically a brand new 2-stroke (Chinese) bicycle engine will not have a lot of compression because the piston rings will not have bedded into the bore. Also a lower oil/fuel ratio will not give as high a compression reading as a high oil/fuel ratio.

If you can see 100 psi on the gauge with a new engine, you are doing ok.

 

[darwin]

Also helps if an engine is new or dry, been sitting awhile to squirt a lil oil into the spark plug hole. Gives you a more accurate reading.