The next time you get the flywheel off look down at the armature plate and with your hands on both sides of it, give it a lateral move back and forth and see if it moves around the center boss a little. These motors tend to develop this problem of a wobbly armature plate after 40 years of use. If yours does move you can easily see that the point gaps are also changing. This I believe causes your mid-range miss. Why in the mid-range I do not know, but in any event if it moves it can be corrected.

To correct this, remove the 5 screws holding down the armature plate and pull it off of the support ring. Remove the plastic bearing and set it aside for now. Put the amature plate back on and move it from side to side to observe the lateral movement. Turn it a little and do it again. It is supposed to not have any lateral movement in all throttle ranges, since any movement will cause the point gaps, and consequently the spark timing to change. If it has movement, remove the armature plate and look at the center boss (the metal material surrounding the drive shaft that is about 1/4" in height, maybe 2" in diameter) that the armature plate’s center opening surrounds. On the sides of this boss, about 1/2 way up its height strike a center punch or nail or sharp screwdriver, with a hammer, at the 12,3,6,and 9 O’clock positions. Do it lightly at first. Replace the amature plate and see if the lateral movement is fixed. Turn the armature plate and check for lateral movement again. If there is still lateral movement, strike the very same positions of the boss again. What you are doing is creating a build up of metal material on the boss to increase it’s outer diameter and keep the armature plate from moving laterally. You don’t want to create too much build up or it will create throttle resistance when in operation but a little resistance is OK. Eventually the lateral movement will be gone. Replace the plastic bearing and put the armature plate back on and connect it down to the timing plate with the 5 screws. Reset your point gaps and check them again at various throttle positions to verify that they do not change (especially at mid range). They shouldn’t change anymore.

With that in mind, if you know the throttle position that you were getting your miss, you could check your point gaps first, then move the throttle to that position and check them again. My guess is one of them or both have now changed at that mid range and that was your problem. If they don’t or you have no movement then perhaps it is another issue. If they do change, that lateral movement should be fixed if you want to have a smooth running motor.