77 KZ750 Twin starter problems

My starter is too weak to turn the engine over fast enough to start. It sounds like it’s struggling. To make sure it’s not a connection problem I tried to start it with jumper cables running to my truck. Didn’t help it. Brushes look ok. I’ve looked for starters online. Are they really that expensive and hard to find? Are there any aftermarket options?

Was the truck running? If so, not a good thing to do. Ed

650ed wrote: Was the truck running? If so, not a good thing to do. Ed

No. And the battery negative on the bike was disconnected.

Could be that the starter motor just needs a thorough internal cleaning. While you have it apart, you can inspect all the parts and decide if it is indeed the problem.
The factory service manual should have a pretty good section on the starter internals.

Took it apart, thoroughly cleaned it, still does the same thing. Turns over slowly and makes a loud whining sound.