I have a Briggs 550EX 140cc ohc motor that I have to hold my thumb across the inlet to keep it running. I think this indicates it has an air leak but there is only the 2 "O" rings on the inlet tube that I can see would allow any air in. I have had the carb apart and I can't see anything in there that could be causing problems, it is a fairly basic carby. Something I'm missing?
