20hms is far too much contact I'd also think. (that is "smoke element" ohms, and the motor winding wire is pretty thin and not quite ni-chrome😉)
Ànother culprit can be the com. plate shifting a couple of degrees on a shaft, changing the timing.
Field coil is just an electromagnet. Weakening and or shifting of the fields strong point on a revolution can reveal issues sometimes. (compare the pull to another fields pull using your "keychain"or something else magnetic near it).
Also makes for fast auto alternator/generator test on the shoulder or in the pits. A functioning alt/gen. becomes magnetic if producing so keys swing towards it when running.
Getting hot real fast also notes a possible loss of wire length/short; by lowering (useful here) resistance, heat comes on very fast.
