If the engine misfires or backfires under acceleration, you might also notice the tach needle jumping out of phase with the engine. This would indicate a problem with the spark box triggering the #1-3 cylinders, which sends the tach signal. The spark box that triggers the #2-4 cylinders sends a signal to the fuel pump relay. If you experience a miss or backfire but the tach needle is stable, try switching the boxes to determine which box needs to be replaced. The damaged spark box will often send a bad tach output or a bad fuel pump output.

The other problem to note is when you have two dead cylinders, either the front two or the rear two. If the pipes on one or the other bank is not firing (or not getting hot), the spark box may be completely shorted and not working at all. You can try switching the spark boxes to determine if the problem changes from one bank to the other. If so, replace the offending spark box.