[Motofool]

Yes, 10 psi will force each piston down with a force of around 46 pounds, if big leaks wouldn't be present.

"Well, I think this is an interesting test that .......tells you where the leaks are and you don't even have to have the gauge to do it.

The test is simple. Apply air pressure to the spark plug hole and listen. You will probably have to take the exhaust pipe and carburetor off plus remove the oil fill cap. Other then that, just listen. You will need at least fifteen or twenty pounds of air pressure. Higher pressure will make the sound louder but too much and you will need to lock the crankshaft to keep the engine from turning. A hissing noise at the carburetor manifold indicated a leak at the intake valve(s). Hissing at the exhaust, leaking exhaust valve(s). Hissing at the oil fill or at the engine breather tube, leaking past the rings.

For air pressure you can use a fancy spark plug hole attachment or just a rubber tip on a blow gun. You might need an extension for these hard to reach plugs too."