1986 5th ave idles ok but pops backfires through carberator

Timing issue. The plugs are firing when the intake is open, thus creting a backfire throughout the intake.

Check all your vacuum lines, in particular the one that goes to the distributor. Regardless of anything else, replacing them is a necessity on older cars.

Next, you probably have that Lean Burn computer, wouldn't be surprised if that's got down the drain.

Get on over to www. Allpar. Com and get into the FMJ-body message forums. There are guys there who can explain issues with that better than I.