Starter burnt after 10 minutes of running

Hello, new member here. I recently purchased a 1984 ZN 750. While I know that it isn't a KZ series, I know it is very similar so I wanted to see if I could get some help cause I have no clue what to do.

When I initially got the bike, it struggled a bit to start but ran fine. After some regular tlc, I try to start it and it cranked very slow. The first starter motor ended up being pooched so I found a used one, brought it to a starter shop who said it worked great.
After replacing the battery and the starter wires, I tried the new starter but it was still cranking slow. The solenoid seemed fine when I checked it. So I pulled the second starter and brought it back to the shop today, and they told me that this one is burnt as well, said it looks like it stayed spinning after I started it (bump started). I checked the starter gear and it spins properly so I don't know what to look for next.

Any help would be greatly appreciated.

It is a KZ,................ the ZN is only related to the 'crusieresque' styling.

...when you say 'replace battery and wires' are you talking about NEW battery or same old battery. I would try a fully charged, healthy battery. A very weak battery would make the solenoid rattle. I wouln't take a starter motor to a 'shop'. I would simply bump it outside the bike with a battery pack, and if it jerks, and spins, is mostly OK.

....I would not buy "stayed spinning after started". It is connected to a free wheeling starting clutch and a running engines spins faster after catching. If you push the button on running engine, it simply free wheels. A starter motor is not designed for continuous running, but a few minutes is OK. Hearing about starters burning is very unusual.

Yes it is a brand new battery and new cables. The cables are a larger gauge than stock so there shouldn't be an issue there. Also, where would you suggest buying an affordable starter from? The only ones I've been finding new for that type of bike is in the 400+ CAD range. I am just worried about dpending that much on a new starter and have the starter burn out again. Not sure as to why it happened.

If you can turn the crank by hand, then engine is turning easy, and the starter can turn it too. A good motor would burn if load is jammed and is kept running too long.

Like I said, I'd simply test it on a bench with a battery pack. It should jerk and turn. If it does not, I'd start thinking brushes. They sell kit to replace those, but of course it requires a bit of labor to rebuild, if the winding is not burnt.

This is a 36 year old vehicle, you have not said much about its history. Other things may need attention, like carburators cleaning, valve clearance check, compression check, air filter, fuel filters, ignition coils, spark plugs, good electrical connection (not corroded). If you bought a 36 year old car, would you expect everything to work just fine out of the box??

Alright, I will put it back together and try a bench test. I know the armature where the brushes contact does look worn but worth a shot.

Also, I was expecting some work but this just got me stumped. Never had too much exposure to starter systems so I've been trying to learn lots. The previous owner said she rebuilt the entire motorcycle, but that was a few years ago.

If the starter works fine, is there any other part of the starter system that I am missing that could cause a slow crank? Also, pic of armature is attached.

The starter comutator copper segments looking "edged shaped lobs" not round shaped.
You need a lathe to bring them back to circular shape,

I am guessing that can be done on any old lathe?

Any Kind of lathe is ok, it must be as round as possible with a minimun amount of copper removed!!!!!

Sounds good thanks! Now to just find someone with a lathe lol. I am guessing the uneven surface causes excessive wear on the brushes?

You you are right and keep attation of the gaps of the comutator segments are not shot cut!

I did the resistance test and all commutater segments check out. Out of curiosity though, what is the mica?

Mica is used as electrical insulator in motors because is hard and very resistant to high temperature. In this case is insulating the commutators bars.

This is a DC motor, not AC, thus the brushes and commutators......there is also a mica like material between the positive connector and the motor's body which is negative (to ground).