I have a cat that is driving me mad! it has a backfire at high rpm only runs fine until you bring up the rpm and then it runs like a bag of sh**. we have changed out the timing belt, no change, the plugs, cap, rotor,wires, even the distributor. nothing. we changed out the pick up on the crank with a known good one and nothing changes. we took a ecm from a good truck and stuck it in and the same thing again. changed out the throttle body, mixer and carburetor same. basically other than swapping the engine that i plan on doing tomorrow we have changed it.

this truck has no cods at all. if you have ANYTHING that my end my frustration so i don't have to work all weekend i would forever be in your debt. the compression test was 206ish across the board.
and yes i have checked the timing marks more times than i care to mention.