Compression Test

[tommy15e]

Gents,

If I do a compression test on my 1987 175HP Johnson, what numbers am I looking for on each cylinder??

Thanks, Tom

[Big Dave]

125psi or so, but all have to be within 10 percent

[tommy15e]

Big Dave,

Thanks very much, I will attempt this for the first time tomorrow.

Tom

[Big Dave]

pull all 6 plugs, open throttle all the way, spin each cyl thru 3 or 4 hits

[tommy15e]

Big Dave,

Got 75 psi on all cylinders +/- 1-2 psi. This is with the throttle in neautral, motor won't crank wide open....what am I missing??

Tom

[Big Dave]

you should be able to disengage throttle from shift either by pulling out on handle at base or pushing button on base......

100 would be a minimum for a useable engine, anything less needs overhaul
fresh engine should be over 135

[tommy15e]

Big Dave,

I have the older style shifters with the fast idle lever. There is no disconnect for the throttle to get into a neautral position. I attempted holding the throttle body open on the engine near the carbs and still get an even 75psi all the way around.

Tom

[Big Dave]

fast idle would be open enough, but 75 is low enough to require an overhaul

[kct11]

try a different gauge for a "second opinion". leak down test is the best way to diagnose a engine. imo, there are too many variables to getting good info from a compression test, unless you are looking for a hole that is way different from the rest. also, there is no std. cranking compression that covers all engines, every engine has a certian range that it has to be in...... check a manual to see what your engine requires.

[tommy15e]

Thanks for the advice, I will try a few different variables. I bought a repair manual but it does not give any info on what numbers to look for. All it tells you is how to conduct the test and what to look for.

Tom

[Costa]

125psi or so, but all have to be within 10 percent


Big Dave, I think he's got one of the first loopers and loopers only run 90psi max compression.

[Big Dave]

I guess I was thinking older unit......