When rebuilding the old 6V starters it is important to undercut the mica between the armature segments, a standard procedure back then. Modern rebuild shops may not do that now. Should be done. Do it yourself. Even using an old hacksaw blade, or other sharp tool, should be cut 1/32" below copper segments.
I had another 6V starter that had a poor solder joint where the 2 field leads connect to the copper bolt that goes thru the case. It was not getting full power, and would run slow.
