I would go with an air leak too. I was having this problem a few weeks ago with an engine and a friend told me to chase it down like a leak in a water heater gas line. I took soapy water, hooked up a long piece of fuel line to the carb, closed the carb all the way then brushed the soapy water around the carb and blew into the fuel line while looking for the bubbles. Shazam, there they were just bubbling away!! Once I found the leak the fix was easy. It can be something as simple as a bad O-Ring under the carb or on a needle valve. Give it a try ans see if you can find a problem.