Reply To: Mercury Mark 20

[Buccaneer]

Yours might be one “known” for jumping into “Forward” when idling in neutral.
I have some notes from the board save while working on a Wizard Powermatic 12,
which was based on a Mk 25.

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billw
US Member – 2 Years
Replies: 2056
Topics: 65
October 17, 2022 at 5:58 am#267768Reply Thanks Quote
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.
billw.

Prepare to be boarded!