Merc 39 Questions

Merc 39

I just got an old Mercury outboard, a Merc 39 3.9 Hp. Itdidn’t look to bad and came with a gas tank, I will eventually use it on a 12ft Jon boat.

Going by the serial number it is a 1968 model (also thedecals)

Picked it up at an estate sale
It cleaned up nice and judging from other items at the house it was probably well taken care of and used as a sailboat kicker


I ordered a new shift lever to replace the broken one and anew impeller and points.
I haven’t tried to start it yet but a couple questionsfirst..
I don’t see a kill button, If I get it going how do you shutit off? Just choke it until it dies?
Also how many parts are shared by this 3.9 with the later 4and 4.5 motors.
I have already learned a little by looking through the partsdiagrams on this site, but does anyone have a downloadable owner’s manual?

I am kind of a sucker for these old projects..

Those are great little motors! Worked on a bunch of them, and they really run super.

There is no kill switch (for some reason), which is really not a problem: just idle her down and choke it. And when you're done for the day, pull the gas connector and run the carb dry--that prevents problems with ethanol gas.

Jeff

start with a clean tank and fresh fuel mixture,clean entire fuel system from inside of tank to carb. Pull plug squirt a little oil into cylinder and turn over by hand first before firing her up in a tank. Check for water exiting the telltale and proper engine cooling

First thing do not spend any more money until you pull the gearbox and inspect the driveshaft where the water pump is. Do not use a hammer directly on the gearbox they can be a bitch to separate from the driveshaft housing just take your time. It can be a jem or it could be a turd if it was a salt water motor be prepared for alot of hours just disassembling it. You can start it with the gearbox removed for short spurts and if it runs good I highly recommend adding a kill switch. A compression test is difficult because the cover has the rewind starter but if you have trouble turning the flywheel by hand with the plugs in it will run if the spark is good. Find out what it needs before you spend any more money parts are hard to find and the gearbox is the most common item that will condemhn the motor to the scrapyard.

The impeller is shot and will likely break up into pieces and clog the motor if you start it up.

Thread #1, owner is getting a new impeller and points installed before startup, nitroberserker definitely follow thread #4 of kimcrwb1, Hope ordered parts do not get wasted!

Thanks for all of your replies.

My parts came today. I pulled the prop off and I was nice and greasy on the shaft.

Also got a Seloc manual to help guide my way.

I planed on replacing the impeller before trying to start it.

No saltwater corrosion is evident and being from Western Pennsylvania it is a good bet it was a freshwater motor.

Looking at the parts blow-up I see an O-ring and shims under the water pump housing I did not buy. I will take it apart first an see what I am up against before buying any more parts.

It looks like some later models did have a kill button, I wonder if I could use one of those? I has a magneto, but I still should be able to rig up a kill button.

Those shims are for pinion height do not lose or damage them they must stay the same and the bottom of the water pump holds the upper oil seal in the gearbox. Replacing the large O-ring is not necessary but recommended if you pull the complete pump. I would try just prying up on the impeller and see if it comes out without removing the complete pump. Put a few drops of dish soap in the cup and then turn the driveshaft clockwise as you push the impeller down onto the key. never turn the driveshaft counterclockwise when installing the gearbox just pull the spark plugs and turn the flywheel clockwise to line up the splines for the driveshaft as you push up on the gearbox.

I suspect the water pump in my 1966 Mercury 3.9 is not working. I understand there is a telltale outlet on the port side under the power head that expells hot water when the water pump is functioning properly. However, on close examination of that area of my motor, I see no hole or outlet where telltale water would exit. Am I missing something or is there another way to see if the water pump is working? Thanks.

Those pumps are elegantly simple hard to put them together so they don't work.------Motor needs a new impeller now and every 2 or 3 years and don't worry about a tell tale.

Kill switches were used on some models and others weren't. I have 4 of those engines, even in some of the original owners manuals I have, its states two options for shutting one off either with the kill switch if equipped or choke the engine to shut it off. Just adjust the idle stop screw to where the engine will stall when twisted fully back to idle if you don't want to choke it to stop. One note for owners of 1970 and later "Phase Maker" ignition models, it is recommended that they always be choked to stop even if they have a kill switch because of the potential of ignition module damage from using the stop switch because this "kill" design opens the circuit instead of routing it to ground.