Engine misfire problem

I recently attended a CPPP and was fortunate to have Ed Watters check me out for flying competence. Our flight started with the usual mag check at 1700 rpm which showed both mags to be working properly. After takeoff and after doing the usual leaning, lean of peak, Ed remarked about the engine not sounding right and he proceeded to enrichen the mixture a little. That did not solve the problem. Ed then suggested that we do a mag check. The engine ran smoothly on the left mag, but when we switched to the right, the engine started to misfire making a popping sound. Next, Ed and I discussed the possibility that one or more plugs were fouled and agreed to run the engine at 75% and rich of peak. The engine ran smoothly and it appeared that plug fouling may have been the problem. A practice landing followed and while on the ground, a mag check was again made at 1700 rpm with both mags appearing to be working properly. At this point I thought we had the problem solved. After getting airborne again, another mag check was made and once again the right mag caused the engine to make a popping sound as if the spark plugs were cross firing. The plane was flown rich of peak for the rest of the flight and the engine ran smoothly. Upon landing, we contacted a mechanic that Ed knew and discussed the problem with him. He asked that I bring the plane to him for troubleshooting. The next day when I brought the plane in, the mechanic advised that he was able to duplicate the problem by running the engine at high power and leaning the mixture. In the end, the problem appears to have been cross firing in the magneto.
After the problem was resolved, I tried speaking with the mechanic to understand why the mag would run smoothly ROP but crossfire LOP. The conclusion was that the mixture affects the current needed to make the spark and under lean conditions, the spark has more alternatives than jumping to the negative electrode of the proper spark plug. This was just a theory from the mechanic and if anyone has some better suggestions I would welcome hearing it.

The spark in a rich mixture… starts to burn easier than a lean mixture. A lean mixture takes a stronger spark. Magneto internal air pressure is an insulator. Higher altitude is not an insulator. (Thus mags are pressurized for T/TN operation)

If there is resistance to a jump…the energy finds the easiest path…in this case…internally


Jim, what you say makes sense. It’s good to know that if this problem occurs again, running ROP is a good way to proceed until the issue is resolved. It’s been a good learning experience. Thanks


Steve…how many hours are on the mags either since new, rebuilt or the 500 hour inspection?

I previously owned a Baron that developed a mag problem while flying enroute from Michigan to Florida. The root cause was misfiring at altitude caused by the accumulation of graphite powder within the distributor which allowed the electrical path of least resistance to fire the wrong cylinder. This was evidenced within the distributor cap with a clear burn path mark.

I understand the powder build up is normal and is another reason for the 500 hour Service Bulletin inspection.

Mike, I probably have less than 250 hours on my mags since they were rebuilt. The shop that recently replaced my mag would not open it to investigate the problem, so I’ll never know if the problem was caused by carbon buildup. As far as I know, mags are not routinely opened except for the 500 hour inspection. Based on the problem you had, it probably would be a good idea to peak inside from time to time and not wait for 500 hours. If that was done, I might have avoided my problem.


What does that mean, starts to burn easier?

And if a lean mixture takes a stronger spark, how do you control that?

You don’t control it. He’s saying that a lean mixture is harder to ignite and burn than a rich one, so it’s less tolerant of ignition system weaknesses such as old cables or worn spark plugs. That’s why a high-power LOP mag check is a good test of your ignition system.