What have I got? Bayer Racing Equipment / Mercury Mk 78 ??

[PrecisionMarine]

Newbie here, looking for info on a recent purchase from an estate sale. This looks to be a 1958 or '59 Merc Mk 78 "Full Jeweled Super Marathon" powerhead with Bayer Racing Equipment tower, flywheel, stacks, and possibly lower unit.
Can anyone assist with more info? What lower unit is this? What class did it likely run, and timeframe it was competitive? Approximate value, current and restored?
Thanks!

[T Chance]

It was probably an alcohol burning class F that ran in the American Powerboat Pro division (Outboard Division then) back in the 1960's. It could be of some value to a collector of old racing outboards, how much, I have no idea. The lower unit is an Eldridge, built by Marshal Eldridge. Jon Eldridge,his greatgrandson races 350cc Hydro with the U.S. Title Series.

[dwhitford]

Yep, definitely an Eldredge gearcase. You can see Marshall's initials -- MHE -- and a serial number stamped on near where the gearcase bolts onto the midsection. Those gearcases had a good reputation for toughness. Steve Dunn swears by them for his 44 Mods.

[Black Sheep]

There are quite a few Quincy components on there. It could be an OEM Mark 75 "Replacement" Block that was modified by Quincy. I would be curious what it looks like behind the rear water jacket...

J-Dub

[PrecisionMarine]

Thanks for the info T Chance, this looks like it will be a fun project. Black Sheep, what mods would I look for in water jacket area that would be different than stock Merc? Wouldn't mods be more likely to porting, or did the Mk 75/78 have a an issue that Quincy addressed? Correct me if I'm wrong, but wasn't the " 'Super' Marathon" indicative of the Mk 78?
Thank you,
Tom

[BabcockBros38A]

A very cool engine, there were a bunch of the 6 cylinders running on runabouts in the 60s through the late seventies in region one. The sound still runs through my mind. As Mr Chance said it most likely is an old ''F'' engine. We would know fairly quickly by looking at the carbs. the main jet size would be huge on the alky version in comparison to a gas version. Another check would be to remove an intake port cover to see if the piston was a one ring piston. AND, in reference to what Black Sheep was mentioning behind the water jacket cover, well he would be looking to see if new higher compression head inserts had been welded onto the cylinders.

Pete

[dwhitford]

Correct on the main-jet sizes: probably about 0.062'' - 0.070'' inches for gasoline but more like 0.125'' - 0.135'' for methanol.

[Black Sheep]

Realistically, the engine should torn ALL the way down. I would bet this was an alcohol engine that ran in the F Class, and if it does turn over, GREAT! But no matter how free you get it to turn over with what ever concoction you use to rinse out the gummed up castor, the needle bearings will not be perfectly clean. And if you do choose to start it, plan on replacing the crank, a couple rods, and welding up the hole you will have in it when a rod jumps out the front! Too cool of an engine to risk it... Just my $0.02

J-Dub

[1100-one]

J-Dub - Your last reply was spot on. This is a cool motor and would hate to see it messed up by not tearing it completely down. Would be cool to know what the history actually was. PrecisionMarine you have done the right thing by asking plenty here to assist you. Pete, J-Dub, dwhitford and T Chance are some good ones and plenty more I'm sure will chime in. J -Dub hope all is well with you!!!!!

[Black Sheep]

Doing well, thanks! Looking forward to running my six with pipes out BOTH sides soon. I plan to have it at Oroville. See you there?

[PrecisionMarine]

Thanks for all the insightful comments and advice. I'll look at carb jetting this weekend, though even if it's a gas motor, I'll still tear it down and do it right. As J-Dub says, no sense in messing up a nice classic.
Back in 'the day', I rebuilt quite a few stock inline sixes, and still have the ring compressors and other special tools.
At this point, I have $400 into it, which looks like it was a helluva good deal.

[dwhitford]

Yes indeed!!

[1100-one]

Glad to hear your going to tear it all down. When done will it be run? Also there is amother site Boat,Racing Facts that most here are on as well. There some old 6 merc guys on there, as well as Looper guys. Just a thought!!! J- Dub might be at Oroville would be nice to see you keep me posted. Remember there is an 1100r boat sitting here lol

[PrecisionMarine]

An informational update: this has Tillotson KA-2A 1-1/8" carbs with .063 jets and 3-ring pistons & unmolested OEM intake porting.. looks like it was a gas motor. I'll commence tearing it down next week, and hope to hear it running within a few weeks.
Any advice on a good match with a late-50's to early 60's period-correct hydro hull?

[jeff55v]

KA2A's were a 1" Venturi. Otherwise, you're assessment sounds right on.
👍🏼

[dwhitford]

Yes, the KA2A carbs have 1'' venturis, but the throttle bores are 1-1/8''. These old Merc-Tillotson carbs have the venturi-diameter size cast in tiny numerals on the bottom of the float bowl.

Elsewhere on this forum recently Jerry Weinant commented that some of the KAs were ''OEM enlarged'' to 1-1/16'' venturi size, even though the float-bowl number was not changed. That interested me! It always pays to measure. And Jerry has far more practical experience with these ol' Merc-Tillotsons than I do.