Not knowing what kind of air cleaner you have, I can't say. Probably wouldn't be an intake leak, as that would affect running and idling.
Also don't think past leaded fuel would affect it (would have long ago been flushed out), but if you're using a lead additive currently, that might do it.
Fuel dilution probably means you're running a bit rich, or you're getting some fuel leakage after running (carb float set too high? assuming downdraft carb.)
