compression test

[Guest]

I have never done a compression test before, but got a great deal on a compression tester set on ebay. So can someone explain how to do one? I think I know how to hook up equipment, but what rpm should you test at and should you start with 1st cylinder and go from there, or does order matter? Also what kind of numbers am I looking for? Doing two 850s and 450. terrylee

[tkent02]

Blow any loose leaves, bugs and dirt off the top of the cylinder head, or they might end up inside the engine. Take all the plugs out, hook up the tester to one hole.
Open the throttle, crank the engine several seconds, until the needle stops going up.
Record the value. It's a better test if the engine is at operating temperature, but it can be done cold.

Test the other cylinders.

All the numbers should be close together, within ten percent or so is great.

Over 100 is good, the higher the better.

If one or more cylinders are low, it could be due to one of several causes, most are cheap fixes, a few are not.

Post the results, we will steer you in the right direction.

[terrylee]

Great sounds simple enough, glad I asked I didn't realize bike is not running and all plugs are out when doing test. Thanks I'll try to post the result soon, I don't think I have any problems but guess only the tests will tell. terrylee

[tkent02]

Without all the plugs out it won't crank as fast, and the results will be skewed, the first cylinder with the fresh battery will be the highest, the others lower.
With none installed the battery will spin them all just fine.

[terrylee]

Thanks for all the info. will try to do in next few days. terrylee

[hampshirehog]

Use the cheapo compression testers with care. I've got a couple - one reads high, the other low (compared to my father's expensive one - a Miller). Fortunately both are consistent in percentage terms across their range and I still use them because of that. The actual numbers on a compression test aren't that relevant within reasonable boundaries; it's the amount of variation between pots that's more important. Plus if you get a test that's on the limit it's worth checking it again with a better tester.

[Nessism]

Engine should be at operating temperature and throttle wide open.

Hopefully you have properly attended to the valves since they will burn if neglected.

If the readings are below 120 or so, squirt about 1 tbs of oil into each cylinder and test again. If the numbers come up quite a bit, you have bad rings. If the number doesn't come up, the valves are leaking.

Good luck

[terrylee]

Thanks guys, adjusted valves last summer. Good to know about tester, since I have no experience with them. terrylee