[GRutter]

The starter on a Model A is a series wound motor. The field windings and armature are wound in 'opposite pairs' (two sets of two windings). Current flows through the field windings, then through one pair of brushes to the armature, through the armature and back to ground through another pair of brushes.

Look at your starter. Two of the brush-holders should be grounded. The other two non-grounded brush holders should be connected to the field windings.

If the starter develops no torque, you will likely have either a bad armature or bad field windings.

It is likely only running at all due to residual magnetism of the poles and armature.