There isn't anything stuck in the muffler that could be blocking the outlet is there? Heard of a similar situation where the carb spewed a lot of fuel and the engine wouldn't start. Part of a cotton towel was lodged in the inside of the muffler blocking the outlet. The owner plugged the exhaust so it wouldn't leak while the engine was sitting and the guy trying to run it (not the owner) couldn't get it to run because the exhaust was plugged. Once unplugged, it ran fine.