Ok, so here is the deal 66 Mustang Automatic c4 from 70ish car. 5.0 from 97 Explorer, untouched long block. 93 a9p ecu and harness, new MSD ignition, cap, rotor, plugs, wires, spark plugs, intake gaskets, new TPS, Timing chain, Temp sensors etc. The only codes I'm getting from the computer are for emissions parts. Car runs pretty strong I know it has a lot more in it somewere, but I'm getting a backfire from #7 at all rpms. I've tried getting rid of it by adjusting the timing witch only made matters worste. The only way to get rid of the backfire is to unplug the spark plug wire. I had the valve covers off to paint them, but dint inspect the stuff too much, but I dint notice anything obviously wrong. I do get some noise from that side of the engine though. I havnt done a compression test yet so we'll see what that says. But if it comes to it what would be a better option using the GT40 heads or the 70s 302 heads that were in it (not sure of what head yet) The old heads would work much better with the headers due to the header/plug clearance issue since the plug wire has a 90* end on it to fit and the boot is still pushed against the primary, could that be causing the problem? We need to get this car ready for a 1500mile trip ASAP so any help would be appreciated.

 

id start by changing the plug wire to another cylinder on the opposite side. to eliminate that piece.

 

forgot to mention I tried many different wires.

 

loose rocker? in either case keep the gt40 heads over the old 70s style.