Compression Testing

[bill-m]

Can someone please tell me when testing compression, how many pulls of the rope would be needed to get a proper reading? Thanks.

[lindy46]

6 or 7 pulls should do it or until reading doesn’t go any higher.

• This reply was modified 5 years, 2 months ago by lindy46.

[fleetwin]

What engine are you trying to check compression on?

[A Hoskins]

A far better indication of effective compression can be observed with a leak-down test. You can make (or buy) a gauged fixture that will let you fill the cylinder with compressed air so you can see if any leaves. It won’t take much pressure to make the gauge drop.

"Fox News" isn't.

[billw]

lindy46 wrote:

6 or 7 pulls should do it or until reading doesn’t go any higher.

• This reply was modified 5 years, 2 months ago by lindy46.

Ditto from me. The key is at what pressure it stop climbing, and whether you have eaten your Wheaties. If your compression gauge does not hold a pressure indefinitely after you stop pulling, then you would need to clean the little Schreader valves. If it falls, back, you will not be able to get an accurate reading.

Long live American manufacturing!

[bill-m]

At present time I will be testing a 1956 Johnson 5 horse motor. What would be a good reading for this motor? I also just bought a 1960’ish Sears 3.5 horse motor that will not be delivered until May. My friend in California said the rope pull needs to be fixed but when turning the prop it seems to have good compression.

[lindy46]

Minimum 75-80psi and even across both cylinders would be an acceptable starting point. Reading may go up after it is run for awhile. Oh, a 56 Johnson would be 5.5hp.

• This reply was modified 5 years, 2 months ago by lindy46.

[outboardnut]

I always ground the plugs when doing a compression test. Thats correct? Right?

[kirkp]

I seem to remember reading that if you don’t ground the plugs you can trash the coils???
Kirk

[Tinman]

I have never grounded the plugs on my omc motors when checking compression and I’ve never had a bad coil. But if it’s a possibility and I’m doing it wrong I need to start grounding them. Curious to know the answer.

[Author]

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