compression test

[BigB9000]

So I did a compression test on a 1973 4hp I picked up.

removed both plugs, installed gauge, and pulled cord a few times. It was a little hard to pull the pull cord, but got 30psi in each cylinder.

decided to wrap a rope around the flywheel and try, as the pull starter seemed to tight to get a good spin on things. did that and I got over 80psi in each.

decided to test my 6hp using the rope on the flywheel method. with pull-start i got 60psi, with flywheel rope, i got 100.

Would the rope trick be a better way to test compression than using the pull starter?

 

["G"]

Re: compression test

So I did a compression test on a 1973 4hp I picked up.

removed both plugs, installed gauge, and pulled cord a few times. It was a little hard to pull the pull cord, but got 30psi in each cylinder.

decided to wrap a rope around the flywheel and try, as the pull starter seemed to tight to get a good spin on things. did that and I got over 80psi in each.

decided to test my 6hp using the rope on the flywheel method. with pull-start i got 60psi, with flywheel rope, i got 100.

Would the rope trick be a better way to test compression than using the pull starter?

If your first try was 30PSI and second was 80 PSI what will the third be? OK so funnies aside... try a wet and dry but you need to have a good spin on the flywheel.

Typically as the piston rolls up the first time it will "pump up" to 80-85 PSI then it should go up from there- at least three compression/power strokes to make a good test.

Next question back at you. Why was it you were doing this compression test? History may tell us more so we can respond better.

 

[BigB9000]

Re: compression test

To see if the engine was even worth buying a new impeller and carb kit for.

History unknown (used outboard)

 

[JB]

Re: compression test

You would have no problem if you would just discard the useless myth about needing a certain measurement of psi. Where the psi fall is more about the technique and the guage than it is about the mechanical health of the engine.

The true purpose of a compression test in a small 2 stroke is to ensure that both (or all) cylinders are making within 10% of the same psi.

The apparent flaw in my argument is that a blown head gasket between the cylinders could cause low and comparable compression. On the other hand a blown head gasket of that sort would produce other dramatic symptoms.