I had a similar problem with the engine that was installed in the RTF Tower 40. In my case the carb, which is a tight fit into the engine housing was not pushed in all the by; thus the "O" ring didn't seal. Loosen up the bolt or screw that holds the carb on, press it down with your thumb - put a dime over the carb opening then push down hard - and tighten the bolt or screw. This problem appears to be quite common. I learned about the fix on this forum. Overall the engine seems to be a powerhouse but quite loud.

Good luck,
Bert