Engine locked up

undefined I recently picked up a kz900 with a locked up motor. I pulled the plugs they looked good, sprayed some kroil down the cylinders and let it sit. I was able to open up the stator cover and use a strap wrench to free up the stuck motor. It didn’t take much movement to do so. The problem is it only spins about 1/2 of a rotation of the stator before it locks up, then I have to spin it back the other way. I noticed with the bike in gear and clutch pulled in the back will not roll. Could this be part of the problem? I was thinking maybe it could be something with the kick start mechanism too. Any ideas? I’d like to pinpoint any problems first before pulling the top end. It looks like all pistons are moving up and down along with valves. Any suggestions would be much appreciated.

You could remove the clutch to isolate the motor from the trans then see if the issue is still present, it could simply be where it was frozen there is a rust band in one of the bores. The other thing that can give these symptoms is a "turned" cam bearing

Spins about 1/2 of a rotation of the stator before it locks up looks like a bad cam timing, bent valve, broken cam chain.
Open the valve cover for a visual inspection.

wrote:

Spins about 1/2 of a rotation of the stator before it locks up looks like a bad cam timing, bent valve, broken cam chain.
Open the valve cover for a visual inspection.

Better yet, pull the head and cylinder/pistons.  "Locked up" is code for corrosion and pitting in the cylinder walls.

I'm thinking the same as Sirocco.  You may have a mis-timed engine where a valve is touching a piston, but you might not be able to check it if you can't rotate the crank.  If it were me I would tear down the engine to examine everything.

For starters I’d be using the kick start to turn the engine over. The stator has a very small relatively soft woodruff key that is easily damaged. You might not notice till after the engine is running. Is the drive chain still one. They rust solid and could be stopping full rotation. In gear with clutch lever pulled in (clutch disengaged) the rear wheel should not rotate. Pull the drive chain, cam cover, and try. If rotation stops then as mentioned you have a heavy rust ring or a cylinder wall completely rusted above the piston. Trying to force this is not going to fix it, but could break a ring. A stuck/bent valve, again force will not correct this. A broken cam chain getting caught up around the crankshaft sprocket, force will not fix. Possibly a broken trans gear or tooth broken off jamming up, force will not fix. Point is sounds like HRM has a good suggestion. Tear it down, you could actually save components.