I was showed this trick to troubleshoot airleaks in weedeater engines and it works with model engines, too. Take your engine and tape off the exhaust port to make it water tight, then place a drinking straw in the intake [make sure the intake hole in the crank is open] and seal around the straw with clay, or anything else that will seal off your straw, you can also use a length of fuel tubing. Now dunk the engine in water, blow into the straw to generate some pressure [it doesn't have to be alot] and observe where any air bubbles are escaping. More crankcase pressure leaks can be found coming out the front of the crankshaft than people realize. A good engine will pass this test with flying colors. Air pressure for the test can also be introduced via the exhaust port, with the carb sealed off.