1979 50 and 1975 500 Mercury Outboard Cross Check

[drsmokehsd]

I have a 1975 Mercury 500 and a 1979 Mercury 50. I need to know if anything from these two motors is interchangeable. I'm mainly interested in whether the carbs are the same. The 79 does not have carbs at all, and the 75 is rusted solid.

 

[WillyBWright]

Re: 1979 50 and 1975 500 Mercury Outboard Cross Check

The 79 uses carbs with integral fuel pumps. The 75's fuel pump is separate from the carbs. I'm not sure what all that would entail. They also use different mounting gaskets, so the mounting patterns might be different.

 

[reelfishin]

Re: 1979 50 and 1975 500 Mercury Outboard Cross Check

I have compared both my '76 500 and a buddies '83 50HP, and they look very similar under the cover. The imediate differences I noticed are:
The 500 has two turnbuckles to secure the outer tin, while the 50HP has only one.
The 500 has a beefier looking lower and mid unit, the 50 HP is thinner looking.
The carbs and fuel system are different

The engine itself looks like it may be the same, with only very minor changes, the electrical system looks very similar if not the same, both mine and his look to have the same control box, same starter, and ignition components.

I have a manual for my 500, but none for the newer motor so I can't compare part numbers.

 

[emckelvy]

Re: 1979 50 and 1975 500 Mercury Outboard Cross Check

The '79 with integral fuel pumps would have a vacuum passage drilled in the intake manifold that matches up with a port in the carb, to provide a vacuum pulse to the fuel pump diaphragm.

If the '75's carbs would bolt up to the '79, you'd have to figure out how to use an external fuel pump such as the '75 uses.

Can't recall without looking at a '79 block whether or not the casting to mount an external pump is still there. If so, you'd either unblock it (or more likely), have to drill holes in the casting for the vacuum signal to the external pump.

You'd have to be real careful about aluminum shavings going into the motor. Maybe drill while using a vacuum to suck up stray shavings.

If the casting was eliminated on the later-style block, you'd have to mount an external fuel pump, and still need to tap into the block for a vacuum source.

At any rate, it definitely would take some adapting to use the older carbs on the newer motor, providing they will even bolt up.

Check the carb bolt patterns; you may find that while the mounting holes on the older carbs are straight across, the newer carbs have a diagonal bolt pattern.

It may even be possible to redrill the intake to fit the older carbs, but again more hassle.

Prolly be a lot easier to pick up a set of '79 carbs on eBay.

HTH............ed