My motors were pulled last year. After re installation one backfires around 2500 - 3500 RPM. I have replaced the throttle position indicator, checked firing order, checked for codes (none), swapped MAP sensors. Compression check is next. Can anybody think of anything else?

Check fuel pressure when it does it, backfiring can be due to lean mixture.

Thanks for the input, I seem to recall that the fuel pressure was OK but I will double check. I don't think its fuel related though because it runs fine WOT.

Have you got a good cap, rotor, and plug wires on them? At 2500-3500 you are pulling "over the hump" so any lean condition or loss/scatter of spark will be magnified at that point.

Pull the cap and lock tite the rotor. should fix the problem

Thanks, I will try that and let you know.

Problem turned out to be the fuel pump and pressure regulator. They were both working well enough to run great at idle & WOT, but not well enough to run when the load got high either coming up on plane or when I backed off a little. Which seems to indicate that the best fuel economy is somewhere over 40 MPH.

My 502 efi developed shutting off this past weekend @ 4000 rpm.... I could run 3900 all day, as soon as I hit 4g's It would shut off... If I backed out fast enough & pumped the throttle about halfway I could save it from stalling sometimes, fuel related? But any ideas where to begin?

fuel pressure problem for sure. had the same issues about a month ago...