My aircraft ("Super -7") has pumped up engine and dual "P" mags. 350 hours. I run NGK BR9ES plugs. Lately, aircraft runs great at lower altitudes, but at 9-11,000 feet, cannot lean to peak as usual without roughness on both mags, no discernable pattern on the engine monitor, only smooth when quite rich. I wrung out the ignition harness, all wires looked cosmetically new, but 4 out of 8 were either open or high resistance. I have a new harness on the way, hope that solves the problem. Anybody have similar issue?
DAR Gary
DAR Gary