i recently did a headgasket job for a friend of mine on his 93 f150 with a 95 junkyard 5.0 and everything went smooth until it went to go take it for a ride. the truck runs flawlessly at idle but when you put a load on it it bogs down and runs rough....then with the brakes applied giving it throttle it starts to pop and buck, backfiring through the intake. i check my timing numerous times and i even set initial timing at three different locations 8 10 and 14. same results at each position...the more advanced the bucking would clean up a tad but the backfire still occurred. i also checked for vac leaks and could not find any. i was in disbelief at some of the things i found on this truck, a real hack swapped the motor and hacked up a bunch of things i fixed. one being the crossover tube on the back of the heads (egr system i think) were just laying in there not hooked up to anything. i capped the back of the heads and plugged the vac lines, the egr valve its self is still inplace and hooked up. i do not think this is a mechanical issue being the heads were just off. i dont really think its timing or vacuum either. i could be wrong tho. i really need some input here, im a heavy equipment/truck mechanic so im not super familiar with sbf.

 

Check your firing order! If you get just the right two wrong, it will kinda seem to run OK, but then fork up just like you describe!
DAMHIKT!
Gene

 

Thanks, i will double check it after work. Any other ideas just trying to rule out basic ****....figure its something really stupid lol

 

The spark plug firing order for a '95 motor is different than a '93, the later motor used the HO order(13726548) versus (15426378) for the original motor.

Also hopefully you did not break the #1 rule of engine swaps... DO NOT use any wiring that comes with the donor motor.

 

The crossover tube at the rear is the Thermactor air injection system plumbing, not the EGR. I agree with the others, you've definitely got the firing order wrong.