Re: Shift linkage
For three and four cylinder engines prior to Mercury lower units: As jerry said: The lower unit must be on to adjust shift linkage.
I like to screw the lower shift rod all the way in, then back it out only far enough to line up the hole with the hole in the coupler. Insert the pin and put in a new (very small) cotter to retain it. The only exception to this is the early 90s lower units with a bellows seal on the lower shift rod. These must have the shift rod screwed out four turns from seated.
Adjustment is under the bottom carb . There will be two nuts on the interlock plate: One above and one below. They can vary in size from 1/2 to 9/16.
SO: First you check for engagement in both forward and reverse with free spinning in neutral. IF it NEEDS adjustment, forward is down so you would adjust both nuts upward, thereby lowering the shift rod to increase forward travel. Do it in small increments until the lower unit locks positively in each gear and spins freely in neutral.
Since the drive dogs have "ramps" to ease engagement, you can check by spinning the prop forward while in forward gear. If the lower unit locks positively, there is enough engagement. If it ramps up over the dogs and drops back with a clicking sound, then you need more forward travel. Since forward gear gets the most load and wear, it is good to bias the shift SLIGHTLY toward forward. Best case is to have the dog clutch just bearing against the gear while in forward. There is space between the drive dogs where the prop will spin freely while in gear. Too much engagement CAN be felt as a slight drag and a very slight sound as the prop is turned between drive dogs.
Do NOT adjust in too much travel in either direction. For example: Too much forward throw will lessen engagement of reverse dogs and possibly eliminate it . It will also put undue load on the shifter saddle (brass) causing premature wear.
The photos show the upper shift rod with the match plate and engine block removed, the brass shifter saddle, the relation of the reverse gear where you can clearly see the ramps on the dog clutch, a gear with drive dogs and ramps, and finally, an assembled gear pack. This pack is from a 2 piece lower unit and is in reverse.
