Help with 1975 Mercury 500

First off forgive me for my lack of boat motor terminology, but I have a 50hp Mercury 500 (Serial number starts with 428XXXXXX). My first time working on a boat motor and I had to change the impeller in the lower unit and put a new water jacket gasket on. That all went smoothly with the exception when I start it up the prop only runs in one direction with no neutral.

I know where the problem lies, I just don't know how to properly put it back together or exactly how it works, so I figured I'd give this forum a try before I spend the money on a manual.

So here is the motor



And the parts in which I don't know how to correctly put back together.





From what I can figure when the throttle is advanced or reversed, it turns the thing with a ramp on it which pushes up on the bar that goes toward the engine, but I'm not sure what that does or how to get it back together properly.

Any help would be appreciated!

Re: Help with 1975 Mercury 500

My engine is very similar if not identical to yours (A 1976 500, 50 -- Serial 441XXXXXX) and I have the original Mercury Marine Service Manual. I took a look at the Gear Housing-to-Drive Shaft Housing remove/install procedures, and while the removal of the Gear Housing from the Drive Shaft Housing is a fairly straightforward un-bolt and slide apart operation, the re-assembly of the two is a pretty complex order-specific multi-step procedure to re-align the upper and lower shift shafts and get the shifting operation functioning correctly. I guess what I am saying is that it seems the proper re-installation of these units really requires a procedural (service) manual to get it back together correctly, unless you are a mechanic specifically experienced with these motors and their assembly (Which I certainly am not!).

Re: Help with 1975 Mercury 500

Grab your Crescent wrench and use it to turn the shift shaft in the lower unit in the Counter-Clockwise direction until you feel some resistance. Then, continue in the CCW direction just a bit more and it'll feel like you've fallen into a "notch". This should be Neutral and to verify this, the propshaft should turn freely in both directions.

Once you've shifted the lower unit to Neutral, shift the control box to Neutral as well and reinstall the lower unit. You may have to move the flywheel a bit in the clockwise (as viewed from above) direction to get the driveshaft engaged in the crankshaft splines. Be sure the ign is "OFF" and plug wires pulled before turning flywheel by hand!!

Hopefully that'll do 'er, check back and let us know how it went..........ed

Re: Help with 1975 Mercury 500

Pliers may help also in lining up shift shaft splines and water tube.A spare human also comes in handy.

Re: Help with 1975 Mercury 500

Thank you for your replies. I'll give this a try today and let you know.

One problem I have, which was my fault is I took the black piece with the ramp off the lower unit (the piece in the second picture) it just slides onto the square rod and can possibly be put on the square rod in 4 different orientations and of course dummy me didn't look to see which way I took it off before removing it. Being able to find neutral should help me in the process.

Re: Help with 1975 Mercury 500

emckelvy and aussieflash, you guys are terrific. With helpful folks like you, I might even have the confidence to try doing my impeller when the time comes! Good luck on getting your motor back together MykeC!

Re: Help with 1975 Mercury 500

With the L/U shifted to Neutral, install the shift cam so that the reverse lockout rod rides in about the middle of the cam's ramp. Since the shift shaft on your motor is square, any other position should be so far out that the one correct position should be evident.

Note that the way the shift cam/reverse lockout rod works is that the cam blocks the rod from being able to spring down, when in Reverse or Neutral. Since the lockout rod can't go down, the reverse lockout latch hooks stay clamped around the tilt pin, locking the motor down.

When you shift to forward, the shift rod along with the cam moves CW and the rod is able to spring down fully, letting the motor tilt up.

Have Fun!...........ed

Re: Help with 1975 Mercury 500

Got it back together and working.

Thanks to all