TOPIC:

Just a thought...

What battery are you using to crank it?

When I was initially attempting a first engine start on my A1 project, I had a similar problem, the bike would turn over with all of the spark plugs removed, but would not turn over with the plugs installed.
I checked everything, wiring, starter, earthing, I was using a fully charged battery out of my Z1 (14 AH), I also tried a fully charged battery out of my car – it wasn’t until I installed a new 20 AH battery (Kz1300 standard) that the problem disappeared and the engine roared into life!!!
Good luck
Andy

Andy has a good point, but we assumed we already passed that and marked the battery as fine. Right? If that's still a grey area, a 11.7V battery will most likely not fire the bike. It's got to be a healthy 12.6+V. So, if that's still a grey area, a brand new battery would be the first thing to attach before more tests. I'd just recommend that and keep the old one spare, be it for DIY home projects (rust removing, testing etc.)

One of the other members here did a great write up on starter overhauls and the problems with a 40 yr old starter and corrosion.
Thought I had it bookmarked however I can't find it.

That was me, like I said, but I can't find it either lol

Jumping the starter "+" post direct from a battery with the "-" via the frame, eliminates any possible issue ahead of the starter circuit. Two ideas regarding this: If you're just "touching" leads or clips to the starter "+" post; that mat be a problem. Not enough contact area to flow adequate current, so try this:. Remove the battery "-" cable from the engine block and use it as the "+" cable by bolting one end to the starter "+" and the other to the "+" on the jumper battery ( Use a Battery clamp). Do something similar for the "-" cable/ground. With a superior connection for "+" and "-" the engine should spin with plugs in. If it doesn't then the problem has been narrowed down to: Corroded contact areas between the starter body and the mounting contact area on the engine block, AND/OR a problem in the starter motor, typically dirty/oily commutators and brushes.
As mentioned , the starter is very easy to disassemble.
The commutators can cleaned-up nicely by chucking the commutator end of the armature in a drill-press. "Snug" the chuck and no harm will be done. I'd suggest a strip of nothing less than 600G to do this.. Use a tooth-pick or something similar to clean between the comm. segments. If you choose to remove the brushes; mark them in some manner so they go back in the same orientation ! This will help ensure full comm. contact which doesn't always happen if you swap the brushes side for side. So basically - Clean the crap out of everything including electrical contact surfaces on the engine block and starter frame.
Hope this sorts out your problem !

Alright, here it is. Just disassemble all, clean, assemble. You might need to make the paper gasket. The gears not shown here, but that's all what they are. Clean them, pack with grease. This starter didn't even move. Totally stuck.

In the last 'assembled' photo you can see the makings on the housing not aligned. I made this mistake and the starter didn't work. After correcting this (just loosened the bolts and turned the housing to the correct position), this almost 40 years old starter still cranks my KZ1300 like a champ.

I also did another starter and despite the outside looking the same, they can be a little different in the sealing area where the gears are. But in terms of service it makes no difference. Just clean, pack with grease, assemble while keeping the commutator and brushes clean. Keep the spacing shims in the same order.

LINK:

kz1300.com/index.php/forum/bike-projects...and-rebuild?start=42