For all the bad-mouthing that Mercs get, you’ll never see one jump OUT of forward, like OMCs do. There’s a strong spring in the prop shaft that constantly has forward pressure on the clutch dog. The engine stays in neutral or reverse, counteracting this spring, in two ways. One is by steps on the shift cam in the lower unit, which is fine when the engine is not running but not substantial enough to hold securely when the engine is running and shaking. The other is by a ball and detent arrangement at the top of the upper shift shaft. You may be able to see it near the aft end of the shift handle. This is a more positive detent than the clutch dog has; but sometimes the ball gets stuck from corrosion. My bet is, that’s where the problem lies. Another more subtle problem comes from wear in the spline connection between the upper and lower shift shafts, and/or a twisted upper shift shaft. These two conditions create a situation where the clutch cam detent and the upper ball detent do not coincide with each other, meaning that the two detents are basically fighting with each other, as to where the neutral and reverse actually are. When this happens, the big spring wins and it shifts into forward. Add to all this, the reverse lock mechanism. It too, has a ramp that has a tendency to favor forward. This ramp is adjustable, so you can try messing with that. But my money is on the upper detent ball being stuck.
Long live American manufacturing!
