Get some WD40 in a trigger spray bottle, saturate the area around where the carburetor mounts, while the engine is running. If it blows white smoke or the engine starts running worse, there is an intake gasket leaking sucking in excess air causing it to run lean and surge.
I generally gap plugs to .030"
Will need the engine model number to look up the correct gap.
One thing i failed to mention is have you checked the flywheel key? It is possible for them shear slightly where it makes it run bad, without no starting at all.