If it shifted correctly before you took it apart then you have your lower unit out of sync with the power head. If your motor is similar to my '77 Merc 50hp then shift the motor into forward and pull the lower unit again. Near the front of the lower unit is a small splined shaft which is your lower shift shaft. There should be a hard plastic or aluminum reverse lock cam on this shaft. This cam is in the wrong position also. Rotate the lower shift shaft until the L/U is also in Forward. If my memory serves me correctly you can not turn the prop counter clockwise while in forward but it will spin clockwise.
Now the reverse lock cam took me awhile to figure out and I don't have the motor in front of me so I can't remember which way is faces but I'll explain what it does. The cam engages a rod on the upper half which activates the reverse locks. If you push on this rod you can figure which direction engages/disengages the locks. You have to put the lock cam on the shift shaft in the correct position so when the shaft turns while shifting it will engage the upper rod activating/unactivating the reverse locks. It took me a couple of attempts while reinstalling the L/U but I finally got it right.
I know this all sounds confusing but its the best I can do without my motor in front of me. I'm getting the boat out today so I'll look at it again. I can't tell you which way the cam faces without pulling my L/U again but I can probably do that if you can't figure yours out. If you have any more questions post here or email me at
kglattha@choice.net
50 hp mercury thunderbolt shifter problems
Linear Mode
