Compression tests on motors

[reelchippy]

Guys is there any way that I would be able to check the compression of an motor myself the with some gauge or device cheers

[reelchippy]

Try to answer my own Question i came across this
15448

[ozscott]

2 strokes dont usually compression test accurately

[blaze]

2 strokes dont usually compression test accurately
please can you explain this statement
cheers
blaze

[reelchippy]

please can you explain this statement
cheers
blaze

Blaze can ask you if the word doc that i added worth anything

[Briannes]

exactly as written reelchippy, you do need a compression gauge that does screw in. all motors have compression some have more than others, a compression reading will give an indication only of motor condition. all must be even. (motor mechanic of old)

[ozscott]

My understanding is that its useful to check variances over the bores, but does not paint an accurate picture of maximum compression. My brother, who is a mechanic explained it to me once years ago, but I cant remember the detail of why.

Cheers

[PWCDad]

I do regular compression tests on my 4 stroke 1052cc marine engines ... they run 11.9 to 1 compression for a static compression of 164 psi. Big numbers for a stock marine engine (Yamaha). It also runs on normal unleaded ... (combustion chamber shape and conservative ignition advance curve see it ok ....to over 10,000 rpm ( cruise all day at 10,000 rpm).

I measure about 2psi difference across the 4 cylinders with a Snap-On comp gauge. Snap-On seems the most accurate to date ... with my 25 yr old gauge and fittings running a close second. I have two Snap-Ons as a guide to compare and see they stay reasonably accurate.

As said in above post .... if psi reading is above minimum set by manufacturer and they are within "differential tolerance" across the cylinders you're right to go ...

I recently honed a 3 cylinder 2 stroke 1300cc motor to get maximum comp for topend speed runs .... only runs half the 4 strokes comp at 6:1 for a static of 91psi minimum. Used same gauge. This engine took my wife to 70.8 mph to be the first female over 40 yrs to run over 70mph on a jetski ...

I do all tests with engine warm ... all spark plugs removed ... and the throttle wide open .... at elevated levels (12:1) the ambient temp has a bearing on the result. Most factory specs are set at 20.0 degrees C.

Max compression reading is indeed a guide to engine wear rates ....

Regards
PWCDad

[Kleyny]

When doing the compression test i do it in two stages
1 dry test all cylinders dry and record readings
2 wet test you add some oil into the cylinder before the test this makes a extra sealing agent for the rings

so if you get poor readings on both tests its not rings but something else (most of the time) in my x mechanic opinion

if on the wet test the compression goes up dramatical then you have a worn rings.

These memories are from 7-8 years ago ( and a few Jimmys)so they could be a little off (but not that far)

neil

ps you can buy compression testers from most auto shops. Ive seen them were you push the tester against the spark plug hole (probably not real accurate if your trying the pull the motor as well)