Outboard Motor Compression Test Question

Just depends on the engine, but you're good to go with those numbers

What I meant is that each engine has a different compression ratio. If you can find the specs, that would be nice, but with them all that close, I wouldn't worry at all

The Yammy 90 in had on my boat had a brand new powerhead on it when I got the motor. When I sold the whole rig 3 years later it had maybe 20 hours on it and I had compression readings of 129, 129 & 128.

I'd say those #s at 200 hours are excellent!

The 70/90 Yamaha 2-stroke is probably one of the best outboards made.

Compression tests will vary from one tool to another, sometimes 15 psi or better. I've taken two brand new in the box snap on compression testing tools and put them on my yamaha, one tool tested nearly 20 psi lower than the other. The harbor freight tool registered 120 psi but the snap on tools were 110 and ~90.

What you are looking for with a compression tester is consistency with the numbers. If you are testing on a cold motor and your numbers are low but even, that's not necessarily a sign of anything unless you know your tool consistently registers high.

Leak down test are for 4 strokes. They do not apply here.

To leak down test a 2 stroke the piston rings need to be above the exhaust ports. I would NOT use this test on a two stroke. It could be done but you run a risk of blowing out the crankshaft seals if the piston is not positioned just right and there is no advantage over a Crankcase Compression Test.


I'm not saying you can't leak test a 2 stroke, because you certainly can. Two strokes use the crankcase vacuum created from the piston to pull the charge into the crankcase and the opposite stroke of the piston pressurizes the charge, pushing it into the intake ports and up to the combustion chamber. There are times when a leak test needs to be done it just requires a lot more than putting the tool in the spark plug hole. You should also never put much pressure, it's something like 6 or 8 psi max. If you are running lean from drawing extra air you should check for leaks at cylinder head, cylinder base, intake manifolds, and crankshaft seals. To do this carburetors and exhaust manifolds need to come off. You have to have all kinds of plugs for different ports, it's a real pain in the arse.