Hello gentlemen, I'm rather new here and my family business owns and operates a 1968 F-850 Super Duty with a 534 under the hood, Because of diagnosis issues on our Mack DM this truck has really stepped up to the plate the last couple of years and has put in very hard work and has not acted up at all (especially since she has alot of miles on her, 2 followed by 6 zeros alot) This motor currently has 235,000 on it and it ran like a top, until it pulled a mountain with a trailer attached en route to pick up a smaller dozer and starting violently spitting and backfiring. We are stumped, did a full electrical and fuel system overhaul and now both cylinder heads are off. Anything we should be looking for in particular? ( besides tearing this pig out and putting in a 71 series detroit) We did a compression test and everything checked out fine and equal, We checked the valve seats and exhaust on cylinder 4 wasn't tight but I dont know if that could/would cause this violent of a backfire. any help would be appreciated

 

Leaking exhaust valve will cause what you are describing.

These engines under load with unleaded gas are prime candidates for burned valves.

If you can get the valves done, you may get some more miles out of it, but without some lead substitute you will burn valves again, especially under heavy loads.

Some engines have survived 5 decades on unleaded. Some died right away. Hard running and high loads are not your friends with an old motor.

Good luck with it.

 

Also check for blown power valves (there's one in each metering block on the oddball Holleys on these engines).