[DCalland]

In cruise I noticed the EGT on the #1 cylinder was running hotter than normal. Figuring a plug was not firing properly I checked the mags and found that the engine ran rough on the left mag, OK on the right. Runup prior to takeoff was fine. This happens only at high power settings in cruise. Below 21 inches MP the engine runs smoothly on either and both mags and the EGTs are normal. Back on the ground a mag check was fine again. Going to tackle the problem this week and looking for thoughts! Figuring it must be the plug, wire, or mag but I don't understand why there would be a difference with the problem in low vs high power settings.

RV 7A
Lycoming O-360 A1A
Hartzell 74" CS prop
Dual slick mags and harnesses, Champion plugs
215 TTSN airframe and engine

 

[Kahuna]

Higher MP produces greater cylinder pressures. This greatly impacts resistance and the plugs ability to fire.
If you were a betting man, bet on the plug.

 

[DCalland]

Great call! Replaced the bottom plug on #1 cylinder and alas! Purrs like a kitten!

 

[Kahuna]

Thanks for reporting back.
Chalk up another good bet.