As a matter of interest, I bought a cheap compression tester.
The Johnson 3.5hp 2st has 60psi but still starts reliably
The 6hp Mariner 4st starts albeit reluctantly when cold and runs fine
Now I cannot seem to get a reasonable compression reading from the Mariner
Is the cause the mechanism that hold the exhaust valve open for easier starting, I wonder, or do I have bad compression?
Sometimes info on the decompression mechanism is hard to find but I've seen one on a modern outboard whereby it is a device on the camshaft which applies an extra lift to hold the exhaust valve slightly open by default at rest and at the speed of pulling over. Then as the engine fires and spins up there is an offset counterweight that throws out with centrifugal force moving this lift tab away from the valve stem so the valve can properly close. So yes with a compression test made by pulling the engine over I don't think you'd ever get a reading or any use.
Sometimes info on the decompression mechanism is hard to find but I've seen one on a modern outboard whereby it is a device on the camshaft which applies an extra lift to hold the exhaust valve slightly open by default at rest and at the speed of pulling over. Then as the engine fires and spins up there is an offset counterweight that throws out with centrifugal force moving this lift tab away from the valve stem so the valve can properly close. So yes with a compression test made by pulling the engine over I don't think you'd ever get a reading or any use.
The Suzuki DF9/15/20 has a decompression system that only gets a brief passing mention in the owners manual. Easy to miss if you skim through the manual.
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I saw on the service manual that the minimum compression should be above 35psi with decompression system in place. Out of interest, I tested it, and it read 25psi and 35psi with a few drops of engine oil in the cylinder. Oil pressure light can sometimes flicker at idle too.
Never fails to start, but it can take a number of pulls in cold weather
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