At what point do I replace my starter?

My 1983 kz550 m1 died a few weeks back. Opened up the starter and cleaned it to almost new looking. I also replaced the brushes. I have put on a new starter solenoid and a brand new battery. After all that, my bike did start up. I ran it for a minute or so. Killed it and tried again, this time with no result. All I'm getting is a click on the solenoid.

Last night I took out the starter and jumped it to a battery charger,... nothing. Off to work.
Today I tried it again. It spun freely this time. Popped it back in the bike,.... nothing. Off to lunch.
I just took out the starter again and jumped it back to the battery charger... nothing.

1. When I try to jump the solenoid posts, I get nothing. The starter should spin, right. Ignition is on.

2. When is it time to ditch a starter for a new/ rebuilt one? Like I said, I just put on new brushes last Wednesday.

3. Where can I get a reliable starter, if needed?

I miss riding. I had just topped off the tank when the starter died. And now gas is at $3.03 here. I'm definitely missing out.

Help and advice is welcome.

Thanks, guys.
LH

For used try eBay. For new contact these folks. Ed

www.motorcityreman.com/kaoe218.html

So I opened up my starter just to double check everything and found one brush spring a little loose. Fixed that, now my bike starts. Now I'm going through my carbs. Why not clean them just for reassurance, right?

Thanks for the link, btw.