Search the Community

Been having an issue with mag drops that doesn't quite fit other topics in the forum. Here is what is going on: Aircraft is a 68 F, I0360 A1A. 830 hrs TSMO. Engine starts immediately, runs smooth on both mags. MAG check results at 2000 RPM: 120 RPM drop on left mag, right mag 400 RPM with roughess. Number 4 cylinder EGT totally cold when on right mag, could not be cleared when problem discovered. Switch on both, now no 4 is hotter than other cylinders. Plug in no. 4 bottom soaked in gas. Pulled mag, had it tested at mag shop, and found mag was excellent. (firing on all 4 with hot spark at 250RPM in test jig). Diode and capacitor all tested perfect. Rotated plugs, retested, still issue with cylinder no 4. Replaced harness, retimed mags, now here is where it starts to get interesting. First runup test, issue still there. However, after 3 runup attempts, engine smoothed out, all EGTs equal at all times, 120 RPM drop each side. Shut down, let cool for 10 minutes. After restart, saw no 4 running hotter than rest at idle, runup failed with 4 going cold again, then no 4 started to fire and followed with perfect mag test again. (120 each side). No 4 bottom plug not soaked, but slightly damp with gas. Mechanic and I are getting stumped. Oil consumption acceptable. (quart per 10 hours), boroscoped cylinders during pre-purchase and found valves and cylinder walls like new. Potential issues could be: Fuel Spider with diaphragm partially blocking fuel to no4? Not leaning enough prior to runup? Not getting engine warm enough prior to runup? Cracked cylinder? Other ideas? Mechanic says to fly for a few hours, then take another look at no4. Thoughts?