Ahah! That's the test I want to know about. Please elaborate, Andy. I want to know about procedure, pressures, duration and the test setup.
Performing a leakdown is easy. Obviously, the first thing to do is look in the exhaust and intake and look for major scuffing. If you see that, you probably already know the answer.
Most piston engine bores wear most at the top of the bore. On a 4 stroke, you can measure anywhere on the stroke as long as both valves are shut. On a 2 stroke you can only measure where the rings are seated on the bore, so it's best to stay at tdc or very near tdc. There are single guage and double guage leakdown testers. Double guage is easier and faster.
Move to tdc.
Jam the flywheel so the crank won't rotate.
Remove the plug.
Hook up the air to leakdown tester.
Set regulator to 100 psi. (makes it easy)
Unhook air.
Screw the hose in the plug hole.
Hook up air.
Verify 100 psi on 1st guage.
Second guage will read leakdown.
So if the first guage reads 100 psi and the second guage reads 80 psi, you have 20% leakdown. On a 2 stroke, that tells you exactly how well the rings are sealing. Doesn't matter what the compression ratio is or whether the combustion chamber is covered with carbon (as long as the rings are free). On a 4 stroke, you can often pinpoint where the leak is by listening for the leak, ie: intake, exhaust, crankcase.
I feel a leakdown test is preferable to a compression test because a leakdown isn't dependent on cranking speed or compression ratio. It is the only true way to check ring sealing without complete disassembly and lotsa measuring. A compression test is ok with a stock motor because you have a baseline - you know what it
should be. If the compression has been bumped though, you have no baseline reading to compare to. It may still run, but you have no idea how much blowby you're dealing with and whether a ring job will gain you more power.
