Thanks, Bax. I've taken the carb off, and checked the mounting block for cracks, but didn't look closely at the engine case. I made sure the screws didn't bottom out, as I read in another post on this engine, and i have, as well as I can determine, a good airtight seal at the carb to block and block to cylinder. I'm going to try two more things; I'm going to try a single tank (16 oz) at 100:1 and see how it runs, and a new plug, as well as looking for any cracks.