My car's been sitting for about late sep early oct. waiting for headers....i finally got them on and hooked everything back up and i started the car, ran a little rough but it had been sitting so i didn't think anything of it. then as soon as i put it in reverse the engine would cut out for a split second and backfire (loud pop) went up the road with light gas and it still did it. but at WOT it doesn't pop but it seems to have a lack of power (off the line more than anything) the only thing i didn't have hooked up was 2 wires that go the AIR switching valve, but i didn't have them in b4. i hooked them back up and it still did the same thing....does anyone have any ideas?

also this may seem stupid, but on my 84 L69, do i have an electric fuel pump in my tank?? i only ask bcuz i was looking as my fuse panel and where it sed F. pump there wasn't a fuse nor any connectors to put a fuse in....

 

Double check the spark plug wires for proper routing and that they aren't shorting out to anything (like the headers). If okay, next check the spark plugs themselves for damage.

If that all checks out, you may have a corrosion problem in the distributor cap or rotor.

 

and if what five7 suggested all turns out O.K. then check your timing. I dont know why it would have changed, but you could have loosened the dizzy hold down bolt on accident when you were installing headers.and shifted it just enough and it's just not continuing to twist around or something... a long shot.