ORIGINAL: opjose
When the engine heats up, your "tight" joint becomes "loose".
Get the engine going and HOT.
Shut it down, and promptly clean it off quickly so it's still warm.
Pull off the back and clean it. Apply some high temp gasket seal compont to the joint, put everything back together and apply Locktite Blue to the screw/nut holding the carb in place.
Remember that the SCREW does all of the work, not the nut ( if I recall correctly on this carb ).... the nut is merely a retainer.
Screw it down tight, but be careful not to strip anything... and let dry.
Carb!? The OP mentioned the muffler halves leaking. Be careful tightening that muffler bolt since it will break if overtightened. About all you can do to stop the leaking is to apply RTV to the joint after cleaning it very well.