Stalling 1965 Merc 39

[merk39]

Recently bought a 12′ John boat with a 1965 3.9 HP Merc 39, every time we take it out it runs great for 20mins then stalls out. Starts right back up runs for a few minutes then stalls again. Changed spark plug, fuel lines from tank to motor and filter. I only use Shell 90 octane off road non ethanol fuel mixed 50 to 1. This didn’t help although motor did respond very well to new plug and filter, started half of first pull. Since the fuel pump operates on crankcase pressure does the motor coming to a higher temp cause the diaphragm to fail if worn out? I’m new to outboards and boating and greatly appreciate any help, the local shops wont even talk about anything except late model. Though they take my money pretty quick when I don’t want advice.

[auldscott]

That sounds like a fuel problem. There are a couple of quick and easy tests you can run: when the motor starts to slow down, squeeze the primer bulb to bypass the fuel pump. Also, try removing the fuel tank cap; if the vent hole under the knob is blocked, the motor will run OK with the cap off.

Third (and I ran into this myself) be sure the line from the motor side fitting to the fuel pump is clean, and take the fuel pump apart and clean it carefully. Tiny bits of dirt can get under the valve discs. Use a magnifying glass to check.

The diaphragm shouldn’t be affected by operating temperature but if it is cracked, holed or torn, replace it. (I have had very good service from Ferguson-Poole Inc.)

Look into the tank. If there is a lot of debris, it is possible that the fuel intake inside the tank is clogged.

Your little Merc should give you a lot of good service. They’re not known as problem motors and they’re not difficult to repair. Of course, there is a vast amount of knowledge in this Club and it is generously shared. Don’t give up on the Merc.

[billw]

Along with what was said above, if possible, I would try another different tank and line, as a test. My guess is that it’s going to be something with the tank and line…

While this may not be the problem, little Mercs of that vintage can tend to overheat a lot. The main reason is an impeller that has bent-back blades. (Impellers, if more that a few years old, also can fail without notice, between the rubber and the brass, which causes NO water; so you ought to be replacing it as a precaution, anyway.) The other way that they can overheat slowly is because of debris….corroded metal dust….in the cooling passages. The next time you run it for awhile, put your thumb on top of the cylinder and make sure you can hold it there. Edit: BE CAREFUL OF THE FLYWHEEL! It shouldn’t be hot enough to burn you. Overheats can cause fuel issues and/or cause the motor to stop because it locks up internally. When it cools, it turns over again. Doing this repeatedly is NOT good; so you should at least check the temp to rule it out.

Long live American manufacturing!

[merk39]

Thanks gentlemen, can’t wait to try these suggestions. I think all of the above applies and I will sort these out before another outing. Also will not give up on my Merc, she’s what attracted me to the boat. I might be a geek for these vintage outboards, couldn’t help myself when a 1965 Evinrude 18HP Fastwin came up for sale. $275 and runs like a banshee. Hope I can give back to this as you said "generous club".

[auldscott]

Keep us posted.

If you change the pump impeller, it’s a good idea to also change the associated seals and o-rings. The lower unit is easy to remove and install, but it’s easier to install the LU if you turn the motor upside-down on a pad on the floor.

[Mumbles]

If it takes twenty minutes for the first stall to happen and less time for the next stall, could the original ignition coil or other components be going south?

[merk39]

I’ll keep you posted, picking up parts on lunch hour today from local shop. Leaving on a 2-1/2 week trip Saturday, going to study breakdown and maybe find a video while gone. Everything will be ready for when I get back, maybe get her tore down before I leave. Probably will.

[merk39]

Thanks Mumbles, I will add it to the growing parts list. Was eventually going to replace those items but should probably do everything at once, would hate to do all the other work and have the same issue.

[seakaye12]

They are admittedly difficult to see in bright sun but you could try an in-line spark checker to see if it is loosing spark when it stalls. Maybe go boating at dusk when it’s easier to see. Maybe a condensor is going bad?

When it stalls; does it re-start immediately? Do you pump the primer-ball before a successful re-start?

here’s what I’m talking about:

https://www.harborfreight.com/90-in-lin … 69023.html

[merk39]

Yes it does start immediately, never tried not giving bulb one pump. Spark checker looks like a must have diagnostic tool and for only $4.99

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