I had same think, turned out the "o" ring from the carb to the jug was protruding up into the air/fuel passage to the piston and plug. Plus it was drawing air thru the gap. I didn't see it until I took the carb off for the 6th time and dicked with the float and new jets. They weren't the problem. The ring is in a groove in the face of the transition piece that attaches the carb to the jug. It seals on a smooth surface on the jug. Check it out, you'll have to take the carb off and stick your head in or a dentist inspection mirror to see if its proturding, it might not be visible and just be out of the groove. Its a bitch to get to stay in when you try and put the transition piece back on the jug to hold the front of the carb.