Re: 1959 58a merc
people will come and post here saying that 50:1 is ok but no because it was not designed to run 50:1. then they say or well it has needle bearings. it was not made to scavange 50:1 gas and oil mix. 24:1
There is no need to perpetuate old wives' tales about fuel mix in these motors. Here's the scoop straight from Mercury Marine, who should know what they're talking about:
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Fuel-to-Oil Mixture Ratio for Older Mercury Outboards
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Mercury Bulletin Number 80-6:
"A. USE OF FORMULA 50-D OIL IN OLDER MERCURY OUTBOARDS
Formula 50-D oil can be recommended (at a 50:1 mixture) for use in earlier Mercury outboards regardless of year of manufacture.
Formula 50-D also can be used in other brand outboards (at a 50:1 mixture), provided that the manufacturer recommends use of a 50:1 oil mixture."
The above was copied verbatim from a Merc Service Bulletins Microfiche.
Since this bulletin was issued over 20 years ago, I'd expect that Mercury's latest oil formulations (and any good TCW3 oil for that matter) would be far superior to the Formula 50-D oil.
If anyone has any information where Mercury subsequently rescinded the above recommendation, please post a reply so we'll know.
A caveat to the above would be that this recommendation was made for production motors. If you are racing or operating a "racing" motor, you of course should run a much richer oil mix.
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Your MK58a is a great old motor, one of Merc's best designs. You can use Hi-Vis Marine Grade gear oil in the bottom end. Wal-Mart usually has a good deal on Sta-Lube or similar and you can pick up a nice little pump that goes in the quart bottle for less than $7.
Use a good grade of regular unleaded fuel and mix TCW-3 oil at a ratio of 50:1 as per the Mercury tech bulletin.
You have one set of points inside your magneto, gap is .008"-.010". Use Champion or equivalent spark plugs, the Champ # is J6C.
Most likely your fuel pump diaphragm has seen better days, especially with ethanol-blended fuels. A replacement diaphragm and gasket kit is cheap and the pump is ridiculously easy to rebuild.
If you have issues with fuel spilling from the carbs while the motor is running, probably could use upgraded carb floats. They're readily available and easy to replace, pull the fuel filter covers off the tops of the float bowls then (2) screws hold each float chamber cover. If you need gaskets for the carbs, Merc has what they call a "packing kit" and this typically is all you'll need. The jets, etc "hard items" except for needle/seat are rarely required to be replaced, just cleaned.
All in all, a relatively easy motor to work on. The only fly in the ointment is the water pump construction, it's under a threaded cover and if the motor's been in salt water, almost impossible to remove. But yours looks pretty nice so I bet it's a fresh water motor. A "pin spanner" should undo the cover if you're lucky, otherwise you may need to beg/borrow/rent the special cover tool.
Definitely would recommend an impeller renew if it hasn't been done lately. You don't want burn your Merc up!
Here's a good site with some service info for your motor:
http://www.maxrules.com/fixoldmercindex.html
The iBoats store may carry some parts for your motor, the following sites do that as well, and also have parts diagrams which can come in handy:
www.boats.net
www.crowleymarine.com
HTH & enjoy your Merc............ed
