Classic/Antique Car Repair: exhaust backfire, engine additives, feeler gage

Question
52 Chevy Pickup 216, Backfires in exhaust on acceleration.  Timing, points plugs
OK. Will engine additives help?  How do I adjust valve / lifter clearance.

Answer
Several things can cause this. Before I went to valve adjustments I would check the distributor cap and plug wires to make sure that I was not getting a cross fire on heavy load when the ignition system is putting out max voltages. Secondly I would run a compression test on the engine. There are adjusting screws on the rocker arms above the push rods. With the engine running and the valve cover off a feeler gage is inserted between the valve stem and the rocker and the adjusting screw turned until a drag is felt on the feeler gage. In my experience engine additives are for the most part marketed to make the person marketing them rich.
Brad