Recently finished up a pretty extensive refurb on my 1650 with one thing eluding me - the looseness in the couplings/splined drive shaft. The one at the flywheel was OK but the two on the back shaft were sloppy, especially at the pump. Have read many of the posts re this and the scarcity of parts I was getting worried, but kept digging and found out about the couplings that Surplus Center had and ordered me up 3 of them (reasonable price too, $13!)
They helped but with the wear on the shaft splines I still had a fair amount of looseness. The new coupler was quite a bit 'beefier' than the originals so I decided to add 3 set screws at the contact points, and it helped -- for about an hour. Two had worked out and lost, and 3-4 more were loose. I rigged up a hex head adapter to a 3/8 ratchet to get them tighter, and it did that -- this time for about 2-3 hrs and were again loose. This time I screwed them with loctite (I was hesitant to do that in case I needed to remove them) but with the loctite and really bearing down tightening them they are apparently holding this time. I believe it would be better to use a bolt, maybe Grade 8 so you could get a socket/wrench on them to really get them tight.