Helping an old friend and his 125

[TommmR]

I'm new to the forum, but only because I'm helping an old friend with his 125. After rewiring his burnt up harness, and doing a maintenance once-over(plug and wire, oil change, grease), the starter turns very weakly. I've gone through the faqs, cleaned all connections, checked voltage drop at each stage, and cranked w/two car batteries in parallel. Still slow cranking and 11.5 v at the starter. I've even jumped the battery directly to the starter.

I've opened the starter/gen, cleaned and undercut the commutator, checked the bearing/bushing(OK), and cleaned the post connections. I've checked continuity on the rotor. The brushes look ok, but the owner says he had it rebuilt last summer by an automotive alternator/starter shop.

Any ideas why it turns over slowly? I get about 6 slow revolutions, and the motor pops each time but not enough to kick it to life. Is it possible the wrong brush material(automotive vs tractor) could cause weak cranking?

Thank you-
Tom