If I may? Have you checked lateral play in the mag plate/crankshaft? Also not brought up yet is mag plate to block. Have a 20hp Merc that was misfiring on both cylinders oddly. I had a current reverse from #1 mag into #2 points. Turned #2 cylinder points gold they were so hot from #1 mag. Remember a twin has two magnetos only sharing grounds only.
Also with suspect coil removed from points & capacitor leads use a 1.5 volt “D” battery to momentary contact primary points lead to check for spark.
A static test on the capacitors is better than no test. Charge cap using your analog meter, then reverse leads on cap in the voltage setting on your meter & the needle should jump up from cap discharging. Look on the underside of the coil very closely for tiny bumps where it may have discharged from running without output not terminated to spark plug lead. The Mag makes energy & it “looks” for the closest or easy path to ground. Remember the wire used on the secondary winding is VERY small & damage comes easy. For testing add a small stranded insulated wire from mag plate to block to electrically bond components please. Good luck & hope this helps,
P.S. Not many check points spring for correct tension, too much & wears rubbing block & not enough & they float.
Jeff
