Im getting a backfire at 5100 rpm's. it is actually 4800 but the tachometer is high. It always happens at the same point. I thought it was the coil and replaced that. It ran perfectly once then started doing it again. I also changed initial timing, it was too high. Fuel is in mid 11's. this has been happening for awhile. Has anyone else had this experience. I cant find anything telling me overboost is on. Boost is controlled by mechanical boost controller. (commercial) ECM is 87 T2 LM