Are all four of the brush holders insulated from the end plate?
How are the two bolts for connections wired, each to adjacent brush pairs, or to opposite pairs?
Have you tried one 12v battery to the field terminals, and another battery, tried both ways, to the armature terminals?
[Whilst trying to understand this topic I found:
https://www.physicsforums.com/threads/four-carbon-brushes-for-a-dc-motor-what-are-they-for.927521/
which has an interesting picture of what an overspeeded armature on a series motor can look like:
How are the two bolts for connections wired, each to adjacent brush pairs, or to opposite pairs?
Have you tried one 12v battery to the field terminals, and another battery, tried both ways, to the armature terminals?
[Whilst trying to understand this topic I found:
https://www.physicsforums.com/threads/four-carbon-brushes-for-a-dc-motor-what-are-they-for.927521/
which has an interesting picture of what an overspeeded armature on a series motor can look like:
