One method I've used in the past to find air leaks is to use WD-40. With the engine running and the throttle closed, very carefully spray (using the little red hose) a very small amount around the carb base. The WD-40 will be sucked up into the engine and the engine will quit running. WD-40 wont hurt your engine either. If you're real careful, you can pin point the air leak this way. Just don't cut your fingers off with the spinning prop.
