Leak tests are done latest at each annual check with aero engines - a bit questionable as there may be just a bit of crap on valve seats spoiling the test. So advice is to do another hour of decent flight and test again. Mostly that does it.
For knowing the point of leak have the air connected to the plug. Then open some inspection cap on the engine and listen to any hiss from inside. That would tell you the piston rings leak a bit.
Then take off the exhaust pipe and listen into head exhaust port, that would tell the ex valve is leaking.
Next listen into carb mouth for hiss, that means the intake valve leaks a bit. Then do your conclusions . . . .
Vic
