I went through this same thing last summer when I corrected mine. That's why I mentioned the 2 different reed blocks earlier. The first metal one did nothing for me (CNC'd). The cast one they produce now fixed it. Used Threebond 1211 to seal everything from the crankcase to the reed block and reed block to the carb mounting plate. Even the carb plate to the carb. My plate is still warped but I made my own gaskets and that sealant I mentioned is the hot ticket. I've worked as a mechanic for 25 years and that stuff has held up better to gasoline than any other product I have ever used. You probably do not have enough run time to see any leakage? You could slightly pressurize the engine w/the carb off and use soapy water. I run mine on a stand and use CRC throttle body cleaner. When it finds the air leak the running engine will dramatically slow down. I do this at idle and use extreme caution, I would not advise it though. Much easier on a single or rear mounted carb.

Did you buy this engine new? Mine was slightly used and the previous owner mentioned they could never get it tuned right to get the serious burble out of it, and to not lean it out anymore (even though the plugs were black and fouled). Gave me a good deal on it and when fixed it now runs great.

Good luck,
Jon