That'd be my next guess. The timing on my old 86 would always need adjusting. It just wouldn't stay in the same spot (it never got to the point of backfiring). Try playing with the timing by turning the distributor slightly. I forgot which way retards or advances. Just remember where it was originally in case this doesn't help. If adjusting the timing doesn't fix it, maybe it's time for a new distributor???? Does the distributor feel loose at all?