I have an interesting problem. I have about 350 hrs on my Lycoming IO-360-C1C and had some intermittent rough running that I have had problems diagnosing. I brought it to a couple A&P's who also could not diagnose it, mostly due to the issue happening only while flying, otherwise everything checks out. The engine seems to run rough only on 1 mag (left), although sometimes both will be affected when both are engaged, but when I single them out only 1 is running rough. I had the mags checked and they came out good. After a year of trying to get to an answer, I have replaced one mag with a E-Mag, replaced all the plugs and harnesses, rebuilt the fuel injector system, and have had everything else checked or replaced. I am now back to the remaining mag.

Last flight it flew perfect for about 30 minutes then started running rough. Selecting each mag didn't initially work, as both ran rough, then I reduced power and the e-mag cleared up so I flew it back on that one only. Strangely at this time the ELT also went off and since I couldn't reach the ELT to turn it off, I had to fly back for 20minutes with the thing transmitting. I am suspecting that the remaining old mag has a defective coil that is arcing once it gets hot. I think the arcing to ground could have affected the e-mag operation until I adjusted the power and then the e-mag timing adjustment circuit corrected. I am not sure how this could have impacted the ELT, other than it is located behind the exhaust a couple feet and could have been jarred by the backfiring. although I didn't think they were that sensitive. Does anyone have an opinion on this?