A sticky valve with cause a backfire through the carburetor. A valve can stick from varnish coming from an old gas tank. Ethanol fuel seems to be a wonderful solvent for 70 year old varnish on the sides of the tank. This dissolved varnish makes its way to the valve stem and gums up the guide, causing a miss. It can be intermittent, such as when the engine hasn't warmed up yet.
