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1959 Mark 75AE RPM Range

t-craw

t-craw

New member
Hey fellas,

Been trying to determine the WOT RPM Range on this motor (59 Mark 75A). I saw
this on a Mercury Outboard database posted by a website:


  • Min WOT RPM: 4000

  • Max WOT RPM: 4600

I thought these old Mark 75's ran higher, much higher RPM's than that. Can
anyone confirm?

I've also tried to confirm the Fuel/Oil ratio as well. I've seen a post
somewhere stating it was 24:1 w/87 octane gas, while another site says 50:1.
Which is correct here?

Any input appreciated.

Regards,
t-craw
 
kimcrwbr1,

Thanks for the quick reply. I'm familiar with oldmercs.com. Bought parts in the past from him and visited his site when trying to get my question answered. Short of sending an email, I did not find the answer on his site but it is a helpful guide/outlet for some things.

I actually came across MercGuy.com and his FAQ. Saw this:

Q. What is the correct fuel:eek:il ratio for my Mercury Outboard?

A. All postwar Mercs except KB-4's, have modern ball and roller bearing internals. This means they will run safely in non racing conditions on 50:1 using modern TCW3 approved oils.

 Perhaps that's correct. Don't really know, although it sounds logical. I'm still quite doubtful however regarding the RPM range quote which I posted earlier. Still think this engine ran higher RPM's than 4600. Thinking somewhere between say 6500 and 7500 WOT.
 
Still think this engine ran higher RPM's than 4600. Thinking somewhere between say 6500 and 7500 WOT.
Click to expand...
Even if the max RPM is 7500 why would you want to run a functioning antique that fast? If my memory serves me correct the leaded gasoline octanes avaiable in 1956 had an octane rating of 91 which rose to 94 by 1966. I would want to run premium octane fuel in your engine. High octane fuel has a slower burn rate which allows the combustion moment to generate higher BTUs vs. the fast burning 85-87 octane fuel.
 
Thanks for the reply,

What I'm trying to do here is match the correct tach with this motor. Got a 59 Wolverine Molded Plywood boat with this 59 Mark 75AE and it came with a surface mount Airguide Model 651 tach but there was not a transmitter for it. The chrome is somewhat pitted and not sure if she works so i started browsing a bit in spare time.

Came across a site that had a NOS Airguide 651A flush mount tach that has a transmitter built in it, for sale. So I started looking at the Airguide Tach Applications for these old motors and it appears there were 2 available for the Mercury six cylinders. One being the 651, which I have (0-8000) and the other a model 655 (0-6000).

This left me with uncertainty as to which is proper for this motor which brings me to ask the question I've submitted... RPM range.

Just trying to keep the nostalgia in this old rig which I bought 5 years ago from a fella in Orlando. Towed it home, ran it in test tank, pulled it off transom and flipped it over for some hull repair. Time has passed, repairs are done and ready to get this motor back on. Have no urge to ever WOT on this old wood boat but ya never know..., ya know? There may be a breif instance where I may be inclined to.

Regarding the racing models for this Mark 75. They did have a Mark 75 H which was the racing model with the short, streamlined lower unit.

By the way..., the fella I bought this rig from in Orlando was really into the old motors, especially Mercurys as well as a few old wood boats. He told me he always ran Aircraft Fuel in his outboards. Any thoughts?
 
I hear ya. Parts can be quite expensive. Rebuilt a 59 Mark 20 some 6 years ago from top to bottom. Before that I did a 68 Merc 200 and bought my parts from Oldmercs.com. However, after several orders from them, I found I needed a seal kit which consisted of a few orings and a lip type seal. Was charged an outrageous amount for that. Kind of rubbed me you know?

Used a different supplier for the Mark 59 but for the life of me I can't recall which it was. They were more reasonable, that I do recall. If I happen to find the site, will post.
 
He told me he always ran Aircraft Fuel in his outboards. Any thoughts?
Click to expand...

The octanes are 100-120 and are too hot for non-racing compression engines. I thought of using it in a truck engine requiring 93 octane and talked to a racecar mechanic who said 100-120 octanes would cook the pistons because of the engine lower compression.
 
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