Impeller wedges into sleeve...

1984 50hp Merc Classic with jet pump.
Last time we used the boat it was running great. Then all the sudden it Locked up. When I tried to start it it would not turn at all. Eventually it would make a revolution before locking up again. Took the boot and impeller out yesterday adjusted shims, put the impeller & boot back on and spacing was all good. Got to the ramp for a test run. Motor started right up jet was operating properly for 1-2 minutes, then it locked up. It would not turn at all. We pulled it out of the water and looked, the nut was still tight and the space on the shaft under the nut was the same so we know it did not loosen up and fall. But the impeller was screwed right into the sleeve? I am wondering if the bearings or something on the inside is shot that allows the entire shaft to slide down allowing gthe impeller to bite into the sleeve?
Please help with ideas of problem and fixes and what to look for stuff?

Thank you, MJO

 

I don't know your rig, but it seems logical that there should be some method that keeps the prop from pushing right into the sleeve. Maybe a picture could put a bit of light on it.

 

no access to digital camera.

I have a 50hp mercury outboard motor. I removed the prop lower unit and added a jetpump lower unit.

Thanks,
MJO