Mag drop

Hi all,

Last week, I flew a cross country from Texas to Colorado in my ELSA RV-12 with a 912 ULS and it performed perfectly. It was parked on the ground for a week in Colorado and on the trip home, I noticed a change in the RPM during runup. When I switched mag B off, my rpm drop was 160ish rpm which is 40 to 50 rpm more than usual and a little rougher than usual. Since it was well within spec, we carried on with the flight and all seemed well until I tried climbing to 9500 ft. It was only then that I noticed that the engine was a bit down on power. Other than that, it was a smooth flight home and I figured I would sort it out this week during the 300 hr Rotax inspection. I still had the original Rotax plugs in so I decided to change those even though I only use 100LL around 25% of the time and always with Decalin. During the inspection, I replaced the Rotax plugs with NGK and as usual the front plugs were sootier than the back. I usually run 93 octane mogas but have recently been flying more long cross countries with 100LL so they did look slightly “leadier”.  I did a leakdown compression test and all cylinders were essentially 80/80 as usual. I weighed the floats which were fine and I removed and cleaned the idle jets while the carbs were off. After buttoning everything back up, I did a runup and found I still had the 50 rpm difference in drop. Having read every thread I could on this site concerning changes in mag drop, I decided to reseat the plug caps that mag A controls so I cut 3/8” off the plug wires on 1&2 top and 3&4 bottom, put a dab of dielectric on the wire, and screwed the caps back on. After running up the engine, there was no improvement. I read somewhere that you should repeat the runup with 50% choke and see if there is a change in the behavior of the problem. If there is, the problem is more than likely the carbs. Well… I tried it 4 times and in 3 out of 4, there was no improvement. Clear as mud. Tomorrow, I was thinking of checking the wiring in the connections to the ignition modules but would love some suggestions as to what I should try next. 

Thanks,

Mark Misage