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.
