Engine rough intermitantly.

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?

Your symptoms are what I dealt with last year. It turned out to be exactly what you described, a "flaky" coil in a mag so that it only exhibited the problem when things warmed up. Replace that mag, and verify your plugs (including the plug resistances) and see if that helps.

FWIW, I just pulled the other mag this weekend for the same reason...I have not checked the coil yet, but it likely is the same thing...same engine...almost exactly a year apart.

The ELT thing is likely a separate issue. I have had ELT's be bumped accidentally and be set off. Especially the older ones that may have been tested a lot and the toggle switch is getting a little loose or something. I had one that I could just tap lightly with my finger on one side and it would set off.

just my $0.02

Sticking valves can cause the symptoms you describe. Check out Lycoming Service Instruction 1425A for more on this. Here is a link for you http://www.lycoming.textron.com/supp...fs/SI1425a.pdf

It can be pretty hard to diagnose engine problems over the intenet, but this is a fairly common problem with Lycoming engines, so it is definitely worth exploring as a possibility.

Dave Prizio

Thanks guys!

I had checked the engine thoroughly for sticking valves, leaks, etc, and found nothing, but that was an early direction for me as it sounded very probable. However, I am about 99.5% sure it is the mag, which I will replace next week. Nice to hear you had similar issues!

Thanks,

Dave