1959 mercury mark 78a

[craveman85]

I just picked up my "new" motor for my 61 alumacraft queen merrie. It's a 59 mercury mark 78a 70hp dock buster. The motor has 110 to 115 psi in all cylinders. It has great spark. It hasn't been run in 6 years but the previous owners took really good care of her. No stripped screws or rounded nuts. No build-up of grease. Thing is super clean. Wiring isn't corroded anywhere. Prop has no dings at all. It was repainted about 10 years ago and it wasn't too bad of a job. It does need a forward starter solenoid which I can use a generic one in it's place. I'm waiting on my impeller before I try and start it but I'm pretty sure it's going to fire right up. My crappy phone pictures really don't do it justice. I was wondering what else I should check on this motor for basic maintenance. I know the owner mentioned a check valve for the water pump but I don't remember what he said about it. Where's the thermostat on this thing? I'm new to the mercury stuff and am looking to get it on the water soon.

 

[emckelvy]

As you likely already know, this is a "dockbuster" or "direct reversing" engine. No Neutral. But if tuned properly, it'll fire off with just a flick of the key and idle like a dream!

I would test for spark before putting fuel or water to it. Likely after sitting for 10 yrs the points have hazed-over. This has a dual-point distributor and the dwell between the 2 sets of points has to be 90 degrees for best operation. There are templates for the degree wheel needed to set the dwell, we can help with that as needed.

Probably a good idea to pull all (6) spark plugs, spray some lubricant in the holes, along with some in the carbs, and pull it over by hand to distribute the lube before cranking with the starter. Better than rolling it over dry.

Only the bottom part of the distributor, held on with (4) 1/4-20 bolts to the distributor drive, has to come off when servicing the distributor. Use caution as the distributor cap may have screws with locking tabs which must be bent down before removing the screws. The side-contact in the cap has to be removed before removing the cap, otherwise you'll break the contact. The rotor pulls off hard, but the phenolic is fragile so don't pry on it & break it. Replacements are expensive! I usually grab the rotor with gloved hand & give it a stout pull. There is a special tool for this but pretty hard to get your hands on one! The plug wires & coil wires screw into the cap and the coil wires just insert into the coils just like a car.

There are 2 rubber flappers in the lower unit, one comes into play when the motor is running in Fwd, the other diverts the water flow in Reverse. Since the impeller rotates in both directions, Mercury did some very clever engineering to ensure proper watering no matter what direction. If the rubber is hardened, the flapper won't seal (or won't open properly), and water pumping will suffer. So it's something to check out when servicing the impeller.

The paint looks really nice but I'm sure you'll want to put some decals on it.

Here's a couple of places with quality reproduction decals for your MK78A:

http://www.americanoutboard.com/deca...er_mercury.htm


http://www.nymarine.ca/1959MCDEC.html

HTH & keep us posted on your project...........ed

p.s. you might want to breeze thru this extensive thread of a fellow boater's '58 MK 78 renovation, a lot of good info that might help you get yours back on the water:

http://forums.iboats.com/forum/engi...-to-revive-a-1958-merc-mark-78-super-marathon

 

[quicktach]

Looks great ...lucky you ! I'd drain the oil from the lower unit and change it on principle. I'd run a magnet through the old oil just to look for signs of wear.