If the mixture is leaned the in flight mag check will often result in roughness. This is how Max Conrad leaned on most of his long distance flights. Turn off one mag, lean to roughness, back to both and it runs smooth. No small airplane egt gauges in Conrads era.
Almost...........but not quite!
If you do a mad check in the air, leaned say 10-20F LOP as I did yesterday, no roughness to speak of, but a power reduction and speed loss. Effectively you have retarded the combustion process a bit. Note I said retarded the combustion process not ignition timing as its fixed at 25DBTDC.
Now if all is hunky dory, the engine will not run rough, unless it is leaned very much so that when on one mag the fuel air mix is so lean its almost impossible to ignite, then when it gets retarded even further its almost quitting.
Yesterday while smooth on both mags I have detected No.2 Left mag plug is ever so slightly off peak performance just by watching the EGT's. The likely cause will be when I regapped them the other day I bet one of the two outer electrodes is a bit closer at some point. Very hard to pick, but I did.
So when doing the LOP MAG/Plug check, you really need all cylinder monitoring, you need to be lean of peak or as lean as you can go and still be smooth (above 8000' if you are not able to do LOP properly) and when you switch to a mag leave it there for 10-15 seconds and watch what happens. Do not be alarmed by EGTS that progressively climb up to 1500 or more. This is OK and normal. They all should rise together and if one falls, you have a crook one and it will be running rough. If it flickers, possibly the plug gap is too large.
Plug gaps must be kept between 16-18 thou when being gapped and ideally after 50 hours they will not be beyond 22 thou.
I have probably left out something important...
Ohh yes........ Hands up who does not have an all cylinder monitor?..........Go out and buy one NOW. If you don't....