Try backing off the valve adjustment a little. It sounds like you've got some intake valves with zero or at least not enough clearance, which is causing the backfires. Also look for carbon tracks inside the distributor cap which would allow a spark to jump forward or back one position on the cap and fire a plug while an intake valve is open. A mean prank we used to pull in high school was to pop a guy's distributor cap and put a few pencil lines inside it- - - - -instant rough running from cross-firing!
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!