It’s been a bit of back and forth to get this running.
Tried it wired into the battery on the car to make sure it wasn’t current limited on the power supply. That didn’t work.
Came back to the shed, stripped the gear train out of it so it was just the armature, that didn’t work.
Pulled the armature out, check poles 180° apart and measure resistance (7.2 ohm on all) measure pole to pole adjacent (6.5 ohm on all) measure Pole to case, all open circuit, measure field winding 2.1 ohm, so that’s all good.
Adjust end float on armature. Someone had been in it before obviously as the plate had been flipped upside down and was allowing about 2mm of float.
So flip the plate back up the other way and then added 2 number, M4 machine washer under it so that when torqued down I have 0.2mm (0.008” feeler gauge) float or so. Then spent a bit of time creeping up the tension on the bolts that hold the field coil in place and also paid attention to the alignment of the armature spherical bushes. I set the armature in place without the brushes and made sure it span freely without hitting anything. It’s so close that torquing the field mountings more causes that to twist in the frame and it then hits.
Then it was a case of testing it with the brushes. Because they are existing brushes i had to work out which way they ran smoothest.
After all that it started to run a bit quieter.
I rigged up the two meters and could see that the current was steadily going down.
With no gears it started at around 2 amps. After about 15 minutes it was down to about 1.7 amps and I could start it and stop it at will by turning the supply on and off. I have since reinstalling the gears and it’s back up to about 2 amps and the park circuit is now working.
it’s only taken most of the week trouble shooting and thinking it through. Relieved to have beaten it.