Purchased over 30 years ago and always had a backfire problem. Seldom drove and some years not at all. Over the years the backfires have gotten worse. However, It runs great when cold with plenty of power and no backfires. Once it warms up it starts backfiring especially on inclines. The warmer it gets the more it backfires and runs very rough with less power. If I let the car set, especially for a couple days it runs great again right off the bat till it warms up. I had a big tune up with a very experienced mechanic and he also converted it over to fuel injection thinking that would take care of the problem. It didn’t. We are both stumped. Any ideas what the problem may be? Thanks! Dave

 

73 -

Kinda hard to tell - since you said it was an old problem.

Use a bore scope to examine each cylinder and make sure there isn't a cracked piston that heats up and causes the preignition.
The stock plastic timing chain may have slipped a tooth.
Uneven cam wear.

Archer

 

I would also look into valve seat or sticking valve. Have you done a cold and hot compression test?

 

I Agree.... Could be the valve seat are getting tight when it gets warmed up..... A good Head re-con should fix that issue...