About compression test

[Tompasio]

Why is it important to check compression with a warm engine.
Are the readings not similar to a cold engine?

[hampshirehog]

The manufacturers always list warm engine tests as standard. The reason is that everything in the engine has expanded (due to heat) and the reading is much more meaningful and reliable.

Having said that, you can make the tests cold and get some guidance from them. If they're pretty close to the warm figures you'll almost certainly be fine but if they're way out then warm the motor and test again before worrying. Some motors change quite a lot and some don't.

[Tompasio]

I did a test this weekend, and the readings was 5 bar (72 psi) at cold engine.The manual says 9-13 bar ( 130-188 psi). but the engine always starts easily,runs wery well,no smoke and the valves was checked for a few moths ago.I guess the tester is out of order and are leaking.I dont think all 4 cyl should have so little compression. How would a engine act if there was so little compression?

[hampshirehog]

A compression tester suffers the same problems as a woman's bathroom scales - you're never sure if the reading is right until you've got a known to test it against. Mine under-reads by a fair bit but I do use it to check consistency across the cylinders so if all your cylinders are reading the same there's probably not too much to worry about; it's unusual for all 4 to plummet together.

Having said that, if your gauge does read accurately, 72psi is low for a cold GSX and you may care to do a warm test. From my experience cold tests on good engines usually run at 100psi or more (others may have different experiences and may want to chime in).

Don't forget, you need wide open throttle for the test - anthing less will give you much lower readings.

Plus, don't get too hung up on compression readings - if your bike starts and runs fine it is fine. I just use a compression tester to aid diagnosis of a fault.

Of course, when the compression test shows low we worry; when the bathroom scales read low the ladies don't..... (I'll be in trouble with that one .......)

[Tompasio]

Thanks for the answers.