Hi Miles,
I like your trouble shooting. I had a similar problem and mine was in
two areas, Carb jet "O" rings were hard as a rock and did not seal, along
with warn shaft openings for the throttle valve kept it from achieving good
vacuum, and the filter screen on the end of the needle valve was dirty and
crushed. This along with a drifting points gap because of the points
breaker plate having slop all kept me from getting much past 55 MPH.
Another good idea is to check the wear on the distributor Cam, 21.94mm
seemed to be what a new cam measured to be. Anything much less could give
you less dwell, poor spark and dirty plugs.
I have also had problems with the starter switch return spring breaking
in half and although it didn't short out it kept the switch from turning far
enough to send current to the starter. A screw driver between the battery
lead and the Black w/yellow stripe wire to the solenoid, showed that the
starter and solenoid were working fine. Changing out the ignition switch
helped a lot, but the wire connections all have to be cleaned and put back
together. Clean terminals is a must, resistance between poor connections
will cause a lot of problems including radio buzz, melted insulation which
eventually could cause a fire under the hood; not my idea of a Hot engine.
Bill