A true life story form our friend Glenn H.
In April 1968 I pulled my Stinson 108, with a 150 horsepower Franklin engine, out of the hangar and did a preflight inspection. Prior to my solo flight I did a ground run-up checking the magneto’s for appropriate mag drop and temporarily applied carburetor heat to assure I had no carb ice. My fuel tanks were full of 37 cent a gallon 80 octane aviation fuel, I had no predetermined destination in mind, the day was clear and the air was smooth. Life was great! To escape boredom I climbed to 4,000 feet applied carburetor heat, reduced power and practiced some stalls. After the last stall I applied power and pushed the carburetor heat button in. To my surprise the engine ran very rough and produced only 1,700 RMP, which resulted in insufficient power to hold altitude. I again applied carburetor heat to no avail. I switched fuel tanks and experienced no improvement. I now realized I had a problem that I hadn’t previously encountered and didn’t have a solution. I surmised that I had a valve or cylinder problem. At this point I assessed my location and ascertained I was too far from any airport and my option was to land in a boggy field. Although a little panicky, I didn’t have any fear for my life, but knew an off-airport landing would probably total my plane and all the resulting problems of aircraft retrieval.
An in-flight magneto problem had never been a topic of conversation among my friends or in written format. To me, magneto’s were something that either worked or didn’t work. For some Devine reason I turned the magneto switch one click to the left and the engine immediately smoothed out. With a sigh of relieve I headed back to my home airport.
My A&P technician removed the offending Bendix magneto and replaced the coil. He ascertained that the offending magneto set up a crossfire which prevented the “good” mag from allowing the engine to perform properly. Magneto’s seldom fail, but when they do, they can do more than just go off line.