Engine turns by hand only 3/4 revolution

[SRAJ_Destiny]

I have a mercruiser 5.0 EFI that would not turn over. I was able to turn it by hand for 3/4 of a revolution then it would thud and stop. I can turn it back the other way 3/4 turn and same thing, stops with a thud. This motor (Port motor) was running, in August on ear muffs, with no issues. Some background info. that may or may not be relevant, I pulled off both drives to paint and perform routine maintenance. After reassembly, put boat in water and motor in question (Port) was running hot (above 170). Only drove it couple hundred yards in idle. Checked thermostat, it was ok but replaced anyway. Still running hot at idle. Put boat back on rack and decided to replace manifolds and risers, since they were due. Still running hot on ear muffs. Then found twisted hose on outdrive. Fixed that and test ran on muffs with no temperature issues. Was going to put boat in water a few weeks later and starboard motor wouldn’t start, had to replace a couple injectors. Fixed that and then went to start Port motor and starter would engage by seeing crank pulley turn slightly. That’s when I turned Port motor by hand but would come to a hard stop after 3/4 revolution. I pulled outdrive all the way off and still have same issue with motor. This motor was run several times on the rack with muffs with no issue before this problem. Boat was never above idle during or after these repairs or got very hot (only in idle with a close eye on the temp). Sorry for long post, first time. Any ideas ????

 

[Bt Doctur]

stuck valve and dont be turning the motor with the crank bolt. Break it and you`ll have bigger problems. Remove the valve cover and rocker arms. You looking for a valve stem not the same height as the others

 

[SRAJ_Destiny]

Thanks Bt Doctur, found broke valve. Just don’t know why yet. Some rust in exhaust ports to manifold, no water in cylinders. Boat has been sitting for a while.