Correct way to test for compression?

[mmisterbungl]

I bought a compression tester yesterday at an auto parts store. The directions say to hook it up and turn the motor over until the gague stops. I did this on several engines I have laying around and got questionable readings. Some were brand new with less than 1hr run time on them and I know should not register so low. I'm wondering if I did something wrong. Do the automotive compression testers give accurate readings? Do I need one specifically for a small engine?

The one I got is this one

 

[jreid50]

Are you running the engine up to operating temp first! By running the engine for a few minutes you allow oil to get to the rings...and for all the expansion to happen! Are you pulling by hand or using an electric starter?

 

[mmisterbungl]

Are you running the engine up to operating temp first! By running the engine for a few minutes you allow oil to get to the rings...and for all the expansion to happen! Are you pulling by hand or using an electric starter?

did not run the engine first, and yes, pulling by hand.

 

[jreid50]

Try running the engine and bring it up to operating temp! then do your test! You wont get as high a reading by pulling by hand! Do you or one of your buddies have an electric starter?

 

[jreid50]

And does your engine have a compression release! If so your gonna get a bad reading!

 

[mmisterbungl]

yes both have compression release. No buddies here that are into minibikes, unfortunately.

 

[Modding_out]

I heard some were that of you can spin the engine backward while it's on the bench you'll by pass the compression release and you will get a better reading.

 

[jreid50]

I heard the same but I dont know if the overlap would effect it!

 

[jreid50]

What PSI readings are you getting?

 

[mmisterbungl]

one one of my clone engines that is just now approaching 1hr use I only got about 60psi.

 

[jesse8931]

Also the throttle needs to be wide open.

 

[jreid50]

YEP Good point jesse

 

[jesse8931]

I learned that when I did my snowmobile and was getting 75psi and was freaking out till I did wot and was getting 130psi.

 

[jreid50]

If you can just remove the push rod from the exhaust side would that eliminate the compression release! I havent done a test on the clone yet! Or would it be better to just relax the alve lash to eliminate the release?

 

[v8ranger]

a leakdown test is FAR more usefull

 

[Modding_out]

On a 2 cycle yes, you right.

But I've never heard of a leak down test on a 4 cycle....I don't even think it's possible, to many place for it to leak out over long term.

 

[Von Zipper]

As has been stated: wide open throttle, at least 6 revolutions and remove 5lbs. of compression for every 1000' of elevation above sea level. The leak down meter is a better assessment of engine condition but lots of people are unfamiliar with them.

 

[jeep2003]

maby just pull it fast enough to rev out the compression release :shrug:

 

[Dudlydoodleright]

What the others have stated is true--another factor in compression gauge reading is small cylinders being tested by gauges made for larger cylinders. The dead air space in the hose and any adapters makes the readings less reliable as the displacement gets smaller. I switched from an auto tester to a small engine tester but then had to put spacers inside the adapters (to take up the dead air space) untill I got proper readings. Still with a compression release you've got a challenge---and yes leak down testing works on 4 strokes!!!

 

[65ShelbyClone]

On a 2 cycle yes, you right.

But I've never heard of a leak down test on a 4 cycle....I don't even think it's possible, to many place for it to leak out over long term.

That's the point: to find leaks. It's standard operating procedure when checking the condition of a four stroke. Cranking compression just tells what the compression is; a leakdown shows where it's going.