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yes it is worth the time and effort. check for spark first. check the lower unit for grease I use John Deer corn head grease. check compression then try to start it . and go from there
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That’s a real nice looking Mark 25. I say go for it. You’ll think you are on a different planet when you do, but hey it’s an adventure.
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Mark 25s are really good performing engines when all their stars are in alignment. But there are a lot of stars. SO MANY stars. They are a lot tougher to make right than OMC products; but think of it as a challenge and don’t be in a hurry.
Even if that cream puff starts right up as it is, do NOT leave the dock without at least putting in a new impeller, and grease in the lower unit. They can fail faster that you can say, “What the….”
Long live American manufacturing!
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My favorite motor ! I learned to ski behind on on a 12′ Starcraft. It’s incredibly fast — my 9′ B-class hydro was on speaking terms with 60mph with just 150lb me and 1 gallon of 98 octane from the local airport. (regular foot)
Some observations:
o The 20 cube Hurricane block is pretty beefy — it raises the center of gravity of a boat. (but it can be bored out to 22).
o Some parts are interchangeable with Wizard motors of the 50’s
o Maintenance on the lower unit is not for the faint of heart and requires specialized tools (or make your own). Turn clockwise to unscrew the rear propshaft carrier.
o The recoil is pretty sturdy, but because of it’s design the flywheel nut is a crazy spiral nut that’s hard to avoid harming when you have to remove the flywheel.
o My go-to place for parts is oldmercs.com. See https://www.oldmercs.com/help_answer.asp?ID=20. At one point I could call and ask for Bob Grubb for his personal input. I don’t think he puts time in at the store anymore. Anyway, you may find yourself purchasing Phelon coils from them.
o John Kwast’s (JohnsOldMercurySite.com) site is good for advice and some parts.
o My Mark 25 has trouble with the throttle arm extension (the one that comes out of the throttle arm at a right angle). The rubber coating on it has deteriorated making the gearshift interlock flakey.
o It runs almost 1000 rpm faster than the OMC 22 cubes of the time, meaning keeping the fuel mixture consistent and at 24:1 (there are those who will say with today’s oils, you can go to 50:1, and you probably can lean it out, but I’d go through a couple of 24:1 tanks before reducing the oil).
o My motor has a compression ratio of 125 on both cylinders. Anything over 100 would be acceptable.
o If it’s been run on ethanol blend your fuel pump could be in need of a new diaphram. I’m working on Johnson 5 1/2 (CD) and ethanol blend ate the inside of the fuel lines. The fuel filter got clogged with the debris so he removed it. Now I’m dealing with rotten neoprene crumbs in the carb, pump and fuel filter…. so – run non-ethanol if you can.
o My motor runs hot all the time. The hydro was responsible for some of that … not much of the lower unit was in the water. But, I think I destroyed part of the seal/grommet at the top of the water tube where it enters the water jacket at it’s lower end…. so that’s something to watch out for.1 user thanked author for this post.
Watch out for the infamous Mark 25 auto-shift feature: Start her up and idle in neutral, then it may suddenly shift into forward on it’s own, especially if you rev the throttle a bit….There are many factors involved in this, between fundamental design and worn, stuck or twisted parts. When everything is correct, this is not a problem. But just watch out for it. I always wondered if a Mercury engineer saw this happen and said, “Hey, it would be really cool if we made a motor that did that ON PURPOSE,” and thus came the Mark 10 and automatic transmission family.
Long live American manufacturing!
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After checking all the things mentioned above, you should probably change the crank seals. Top one is fairly easy after you remove the magneto. For the bottom, you have to remove the power head and drop the bottom bearing carrier to get at the seal.
Use heat to drop the bearing. Not that hard. These are great running motors. I have 6 that run well.1 user thanked author for this post.
Dad bought a Mark 25 new in 1958. It fouled spark plugs on about every 3rd or 4th tank of gas unless we used Mercury Quicksilver oil and “white-marine” gas (additive-free, and lead free) that you had to seek out back in the 1950s. We water-skied a lot with it every weekend in the summer, and it required frequent tweeks, and things sometimes came apart, like the day the recoil starter disintegrated and knocked two small holes in the top housing – fortunately under warranty yet!. He sold it a year later because it broke down so much. I like to think he just got an isolated lemon. I still think they are nice motors, but I wouldn’t leave the dock without oars. They are fast for 20 HP, but take along earplugs! That’s my 2 HP opinion on that model.
Good Luck!
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