Hmm . . . pilot air setting affects mixture across the whole range, but if you richened it to slow the idle down (?) by screwing it IN, then I'd have thought it would burble at you at low revs, or even '8 stroke', and make the plugs a bit black up the range. But not maybe cause a misfire. If you UNdid it to weaken the mixture until the thing slowed down - tho usually revs will rise a bit, not drop off, as you start to weaken - then it could have affected the thing I s'pose? But it would probably have been spitting and coughing on the throttle at all settings if you'd unscrewed it that far, and maybe become a pig to start as well?
Too many variables! I'd start with going back to maker's on the air screw - turn and half out from fully in maybe, depending on what carb? - set the idle by the other screw like you said, and then see if things are better, worse, no change.
Thinking more about what DuTch suggested, if you have a dark corner in the shed, you might just be able to see any stray arcing from an HT pick-up, if there is any. Usually it'll be the one on the front, cylinder side, of the mag, and any sparks will show up quite well in the dark! But then again, if it needs a heavy load to make it misbehave, it's harder!
