Horn won't work

[700rpm]

I have a horn I picked up at a swap meet about a year ago. It wouldn't turn when power was applied, though it did turn manually. I noticed one wire on the armature was broken, and soldered it back on. Right now I have the motor assembly apart from the projector, and the adjustment screw backed off. There is no contact with the diaphragm, but the pressure arm is still attached and pressing on the ratchet. But it still won't turn! When I attach a couple pigtails to the contacts to complete the circuit and actuate the horn, it makes a slight effort, and might even rotate occasionally and very reluctantly about 1/12 of a turn. I've had it completely apart, and everything is lubed and looks like other horns I've cleaned up and worked on. But obviously I'm missing something.

Not being a strong electrical guy, I don't know what to try next, or what to suspect, or what to look for. Your suggestions?