Thread: Old 60's Mercury 3.9HP Help please

Pictures, Model and serial numbers would be a good start.
You can go to Marine Parts Plus Mercury Serial 39 1 CYL 815749 and get a parts breakdown chart to help see the parts and the order that they go together.
As for the gear being in backwards ..... I am not sure it can be assembled with it backwards.
When you say the motor fired one time......how long was it running before you tried to shift it?
Hope this helps some.

Any idea which part I need to download to find the pieces I'm looking for? I only have limited space on my tablet for any new downloads.

I have a Clymer's manual that covers that motor. However, there is nothing in it about the shift linkage.

It sounds like the shift shaft in the lower unit is not in the correct position. I have not had one of the small lower units apart but on the e bigger ones if the shift shaft was rotated past the forward detent on the shift cam it will not shift no matter how you adjust the lever on top.

You will have to pull the lower unit off to correct this.

Once you have the lower unit off the shift shaft is in front of the main drive shaft and sticks up a couple of inches. If you can rotate that shaft back and forth with your fingers it is out of sync completely.
If this is the case you will need a pair of pliers to grip the shaft. It will require little bit of force to rotate it.

The next step is done with you looking from the top end of the crankshaft towards the bottom of the lower unit.

Rotate the shaft clockwise about an eight of a turn or so. You should feel the shaft click into a notch in the cam.
This is the first of the 3 detent spots on the cam.

Once you find the detent only the shaft only about a 1/4 turn as it does not move very much to shift gears.

Then rotate it slowly further clockwise you should feel the other two detents. STOP as soon as you feel the third detent.

Rotate it back counter=clockwise to the next detent and stop. This is the neutral position. The prop spins freely both ways.

You then can move it clockwise to the next detent and it should be in forward gear. Check this by rotating the Drive shaft clockwise to see the prop move.

I usually leave it in the neutral position and put the gearshift in neutral when putting the lower unit back on.

If you can send pictures of what you are working on would help us a lot.

Hope this helps you.

PS You deleted your message about what you were doing from the post somehow. You should repost it so everyone can see it.

PREVIOUS POST BY airdreams1


I only let it run about 30 seconds or so before I tried to shift it which probably was not long enough for it to get warm.
I'm 99 percent sure that the cure is in backward or the shaft is upside down or something. The gear is up in the air and not messing with the other one until you turn it all the way to the maximum and then the gear starts to mesh for about 12 and almost pushes it over to forward or reverse that part I'm unsure which direction is what. I can tell that it has been taken apart because the pen that stops things from moving was loose as well as the outer retaining nut for the handle. Also there seems to be some sort of bushing that's just loosely laying on the shaft and not pressed into anything

Important note to my previous post.
I just found a note that the shift shaft on models before 1971 may operate in the opposite direction than what I said earlier.

You may have to reverse the directions of turning for it to work.

Just don't apply a lot of pressure when trying to rotate the shift shaft. Parts for that motor are hard to find.

Sorry the confusion. dfar