Update on engine
issue: I now have a good idea what happened. When the last service
was done, the decompression level was disconnected from its control cable and the cable applied to the throttle instead.
Unfortunately, the decompression lever rotates through 180 degrees. In the on position with the cable pulled, the screw ends passing through the control rod prevent the valves fully closing. In the 90 degree position the decompression stops are disengaged. The cable would normally be fully pushed in for this position. However without the cable the lever will continue to rotate to 180 degrees bringing the screw heads with their lock nuts in range of the rockers. This is certain to be what happened because the screw heads are chewed out.
Then the vibrating force on the screw heads caused the free end of the control lever to vibrate, eventually snapping the control lever at the midpoint where the central bearing with the friction fit blot is located. The broken off rod then bounced around inside the rocker housing, jamming the rockers and bending the push rods. There is some damage inside the housing indicating this is what happened. The two valve clearance adjusting screws associated with the bent push rods were also so badly chewed out I had to use a hacksaw to open up the slot for the adjusting screwdriver.
I think this is a design fault in the Kubota Z600. When assembling the system it would be hard to screw the stoppers into the decompression control rod if they could not face out of the cover, so they're allowed to sit in that position even though if they were in that position during operation it could cause the problem that i had. [To adjust them, they are 180 degrees from that position and there are special cover plates that can be removed to perform the adjustment.]
There should be a lock that can be put in place after assembly to prevent an unsecured decompression control rod turning into the dangerous position. Relying on the control cable seems a bad idea.
Since I only have a decompression control rod i will remove the stopper from it and put it back in place (to allow the seal to prevent oil
leaking out). However I will have to secure it somehow to prevent it falling out. I could also just put a cover over the hole. Repairing the control rod looks tricky. A weld would add to the length and move the stopper out of position. The ends would have to be ground down to account for this. Not sure it's worth it. Decompression lever is a nice tool but I guess I can live without it.