I found a MM35 and hooked it up.

The wire feeds super slow out of the Tweco #4 gun. The wire feed knob works, but even the 100% speed is too slow. It does not appear to be a problem with the gun liner. Feed motor brushes look good.

I poked around the inside and 2 of the large diodes are shot, one open, one shorted.

I'm replacing them tomorrow, do you think that the diodes somehow will also fix the wire speed problem?