The lower unit has a unique feature that may be unfamiliar… There is a spring loaded brass cone which is supposed to slip if the prop hits something. Two problems can show up with this. First is that the cap that tensions the spring has loosened, and the assembly slips under load. The second is that the cap splits and there is no tension on the spring at all. I’ve seen several of these with broken caps in the larger K series motors of the same era, and a few in the A series. It’ll take a while, but you should probably go through the whole lower unit.
The large aluminum nut on the front of the gearcase is the locknut for the gear mesh adjustment. With the nut loosened, you should be able to adjust gear mesh by turning the slotted carrier with a screwdriver. When you get to the point where you are reassembling the lower unit, get back to us if you have questions.
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