Mercury 78A? Anyone know anything about it?

[ErrorS]

I picked up an old 15' runabout for $300, hadn't even looked at the motor before I bought it, I just wanted a cheap boat to hold me over until I could get something a bit nicer (about two months)

Anyways, the motor looks like it's in good condition. From what I'm able to find out the motor is from 1958? The title for the boat says 72', though I'm not sure that's accurate. (possible the boat was issued its first title in 72'?).

I'm really wanting to know about the motor though.. I'm going to use the boat and I have very little experience running boat motors. I dunno the fuel/oil mixture, how to start it (I read it has no neutral), etc. .and because it's so damn old I can't find out any of this information.

 

[tashasdaddy]

Re: Mercury 78A? Anyone know anything about it?

what is the serial number on it? sounds like it is a "dockbuster", it runs clockwise and counter clockwise. there is no neutral. they are rare these days.

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

where would I find the serial? I got the model number from the front cover.

 

[tashasdaddy]

Re: Mercury 78A? Anyone know anything about it?

should be in the area between the transom mounting clamps.

 

[steelespike]

Re: Mercury 78A? Anyone know anything about it?

I don't know if there are any reliable serial number charts for that old.
Mark 78A is the 59 70hp Super Marathon six.I should be able to dig up some more info.

 

[Chinewalker]

Re: Mercury 78A? Anyone know anything about it?

Manuals, parts and info at www.oldmercs.com...
- Scott

 

[Scaaty]

Re: Mercury 78A? Anyone know anything about it?

If its in good shape, it probably has more collecter value. The old Dockbusters were super looking motors, but, well..a bit "finicky"
Oil? I'd run heavy 25-1 to start out if its been sitting. If me, I'd try a look over at the AOMCI site..
http://www.aomci.org/aomc.htm
Post in "Ask a member"..about collecter value..

 

[steelespike]

Re: Mercury 78A? Anyone know anything about it?

If its in good shape, it probably has more collecter value. The old Dockbusters were super looking motors, but, well..a bit "finicky"
Oil? I'd run heavy 25-1 to start out if its been sitting. If me, I'd try a look over at the AOMCI site..
http://www.aomci.org/aomc.htm
Post in "Ask a member"..about collecter value..

My old after market service manual says 20:1

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

Frankly, i'm a bit worried about starting it up.. I found a thread on another forum that had quite a bit of information (more for boat engine tech-heads) and this engine seems complicated as hell.
I drive old vehicles because they're so much simpler and easier to fix, doesn't seem to be that way for this old engine.

I can't wrap my mind around the way it works.

My throttle lever looks like one from a typical boat, push it forward or pull it back.. but on this forum they're talking about a Z shaped control box that has different switches to start the motor in reverse/forward rotation.

Then to add to all of my confusion, I pulled plugs from the lower section to fill/check oil and it's packed full of thick grease? I only vaugely remember how to do it on my old boat. The top plug was labled 'flush', there was a much smaller one between them with no lable, no idea what that is. I'm scared to even try to dry-crank the motor because of the stupid grease. I couldn't get any oil through either hole, it's really packed in there.

I can't find the water pump inlet either.

It's a shame, this motor looks like it's in really good condition. there is a tiny bit of wear on the lower unit (nothing serious, I've seen 00's with more nicks), the motor has been spraypainted black but otherwise it looks nice, especially inside the cover.
Maybe I'll put up some pictures tomorrow

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

Sweet, found an answer for the lower unit question.. on that eastcoastmarine site:

"Gear Grease- 105 Lubriplate replaces original gear grease in all 1, 2, & 4 cyl. Mercurys through 1961. This heavier lube should also be used in non-shift motors of other brands. 10 oz.

Note: Modern 90 weight gear lube is generally unacceptable in these engines. "

that's good news.. and the grease looked shiny and new, no water whatsoever. (hope the small amount of gear lube doesn't cause issues)

I swear I think this thing was pulled out of a garage it had sat in for 30 years, spraypainted black and attached to this boat prior to me buying it.

edit: Oh Good Lord, a distributor CAP is $140.

 

[steelespike]

Re: Mercury 78A? Anyone know anything about it?

While the motor does have some unusual features it is basically 3 two cylinder motors stacked up.I believe 3 carbs,3 fuel pumps.If it is a dock buster it should have some device for starting in forward and reverse neutral is simply engine off.I believe there is some other complication under the hood possibly two distributors.Very difficult to operate thus called dock busters.
If it is a full gearshift it will have 2 levers.throttle and shift.
I would bet there are some collectors that would give their eye teeth for a
good dock buster.For that matter any Mark 78A only draw back is size and things like $140 dollar distributor caps.

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

The control box is just a single throttle control with a button on top, looks almost exactly like what I had in my old boat (minus shifter, just remembered my old merc 900 had it)
I'm worried someone dropped in a control box from another boat, realized it wouldn't work and then sold it to me. The throttle plates don't open all the way with the throttle moved all the way forward, so that sorta adds to my worries.

I think I'll get into the control box to see what's up, pulling the throttle lever back retards the distributor (or advances with motor in reverse.. oh god, this is all so weird) and doesn't open the throttle plates fully, so it seems like that's all set up right for reverse, though I don't see where the mechanism for actually reversing the engine comes from.

I think I found the water unlet under the lower unit, definitely wont be able to put muffs on but I think a hose in a bucket might work. I'll drycrank it tonight to see how it turns..

and it's one distributor, I guess the weirdness is in the distributor itself. I noticed some play in the mechanical advance, thought it was just worn but I read this is apparently how it handles the engine reversal.

and to confirm, yes there are 3 carbs. Didn't see the fuel pump.. there are 2 ignition coils though.

getting a bit worried, I'm starting to really like this motor but I can't afford another old project. My truck's build-up is killing me and a distributor cap for that thing is only $8, heh.

 

[4.0l sahara]

Re: Mercury 78A? Anyone know anything about it?

Post up some pics of it.

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

Gonna have to wait until tomorrow, just plain forgot today.

I did post an ad on craigslist, "Gauging Interest" to see if anyone might want it. More than the money, I think I'd just feel guilty using this motor without intention of fixing it up.

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

http://picasaweb.google.com/errorsx/TruckBoat

not sure how picasa works, new to me.. but here are some pictures of the motor and the boat. There are some pictures of the controls in there as well.
I didn't organize them, so there are some miscelanous of my truck and a few other items.

 

[jimmbo]

Re: Mercury 78A? Anyone know anything about it?

It's a dockbuster. 2 fuel pumps on starboard side, 2 starter solenoids. 1 HUGE rectifier

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

anyone got any hints on getting it started? ended up flooding it trying.

first ran old, bad gas through. Let it sit with the plugs out, cranked it for a bit to dry it out.. then put in proper fuel (about 20:1) and cranked, never did fire.

Ive got the key, button on the throttle lever and the throttle. The button cranks the motor, the key in the 'run' position closes the choke.. not sure if I need to put the key in the crank position to start it or not.

anyone have any idea about the basics? about how much should the plugs be gapped (they're at about .15 now, which seems awful low)

 

[captk]

Re: Mercury 78A? Anyone know anything about it?

we called those motors "aim and fires". Point and shoot and hope your headed in the right direction. The starter is double wound, and when you shift to reverse and hit the button on top of the shift lever the motor starts in the opposite direction. shift to neutral the engine quits, shift to fwd. push the button and go. Gave my manual to a guy I havnt seen in a while. Try to get hold of him and get back. Good luck with it. Bill

 

[ErrorS]

Re: Mercury 78A? Anyone know anything about it?

I'm sorry, I got a bit lazy.. I was hoping just because I wasn't familier with boat engines there was some magic button to start it that I was missing.
I started troubleshooting like I would a car.. pulled one of the plug wires and got no spark at all, the wire going to the ignition coil was held together by a splicer which fell apart when I touched it, so the motor is going to need a bit of rewiring.

Going to try replacing that ignition coil wire that's corroded to see if it gets me spark, if not I'll rewire the whole thing.

I've already fallen in love with this stupid engine.. I see friends and family, first thing I tell them is "I got a boat! and guess what, it's from 1958! the motor doesnt' even have a gearbox, cool, huh?"

 

[cjack56]

Re: Mercury 78A? Anyone know anything about it?

I'm sorry, I got a bit lazy.. I was hoping just because I wasn't familier with boat engines there was some magic button to start it that I was missing.
I started troubleshooting like I would a car.. pulled one of the plug wires and got no spark at all, the wire going to the ignition coil was held together by a splicer which fell apart when I touched it, so the motor is going to need a bit of rewiring.

Going to try replacing that ignition coil wire that's corroded to see if it gets me spark, if not I'll rewire the whole thing.

I've already fallen in love with this stupid engine.. I see friends and family, first thing I tell them is "I got a boat! and guess what, it's from 1958! the motor doesnt' even have a gearbox, cool, huh?"

Hello-

I know, I'm a bit late to the party :redface: but I can help you with all kinds of answers about your Mark 78a I hope you haven't gotten discouraged and given up this last year!

1958 was the first year these were made by Mercury. They called them "Direct Reversing", or as others have mentioned; "Dock Busters". As others have also said (and you yourself have noticed), there ain't no Neutral, just Forward, Reverse, or off.

You start the motor in one direction to go forward, then to go to reverse you move the lever back and hit the starter button again to start the whole motor rotating in the OPPOSITE direction! This was Mercury's answer to their belief that the gearbox (lower unit) wasn't strong enough to stand up to a six cylinder engine!

The reason they were called "Dock Busters" was if you approached the dock, tried to start it in reverse and it didn't fire in time you rammed the dock.

Video of a Mark 78a in use:

http://www.youtube.com/watch?v=hU-hZOnvg1E

Overview of the engine:

http://www.eastcoastmarineservice.com/mark78a.htm

OK: some links to a LOT of guys who know lots about these engines (repairing, parts sources, setup & tuning, oil mix, etc etc.)

John's Old Mercury forum: http://www.pfs-ware.com/talkshop/
Fiberglassics: http://www.fiberglassics.com/forms/forum818C/
East Coast Marine: http://tinyurl.com/cwqbef
Antique Outboard Motor Club Inc: http://www.aomci.org/

These are just a few, there's many more.

From the photos, you do have the right control box there. That model should have two micro-switches in the box to control which direction the starter turns.

For troubleshooting the ignition, try here:

http://www.maxrules.com/oldmercs/timing/1.pdf

Internal wiring harness:

http://www.maxrules.com/oldmercs/Wiring/0_1965a/5.pdf

External wiring & control box:

http://www.maxrules.com/oldmercs/Wiring/0_1965a/1.pdf

Carburetion & fuel: http://www.maxrules.com/carbindex.html

If you need more help (encouragement etc.) I can be reached through this board, or on Fiberglassics as "Captain Jack".

Smooth sailing!

The Cap'n