Compression tests and a bit more 1954 Cub

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Compression tests and a bit more 1954 Cub

Postby RandyM » Sun Sep 20, 2015 7:53 pm

Today I had some problems keeping the Cub running and and found a really rusty cruddy gas tank. It's now time to get it cleaned out better than I did the first time! But that's not my question -

While I have the tank and hood off, I thought I'd recheck the compression. First, after having just been running an hour prior and leaving the other plugs in, I recorded the following: #1 - 78 psi, #2 - 75 psi, #3 - 78 psi, #4 - 89 psi. After this, I pulled all the plugs, and retested to get: #1 - 95 psi, #2 - 90 psi, #3 - 91 psi, #4 - 100 psi. These values are average of two-three tests on each cylinder. (I did not give the cylinders a shot of oil to do a wet test.)

I found carbon build ups on all the plugs (Autolite 386s with an estimated 35 hours run time). Valves were checked (engine cooled at this point) and all gaps were measured at 0.015.

There had been blue smoke at one point but I've run Seafoqm through it once and that has mostly cleared up, although today there was some when I started it after it has been sitting for a few weeks.

I know the compression is low, but which seat of numbers is the correct set? Do they indicate an immediate need to do an engine rebuild? Will a different plug reduce the carbon build up?

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Re: Compression tests and a bit more 1954 Cub

Postby Cecil » Sun Sep 20, 2015 8:12 pm

For being dry for the test those numbers are not that bad. If you had the plugs in there when you did the seafoam that could be why they had carbon on them.

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