Lower Unit Issues

[Bob in Tennessee]

1977 50 Hp Merc 500
Original lower unit destroyer on rock. Replacement is having issues shifting and running. Engine starts and idles fine but when shifting into forward, engine dies repeatedly. After several attempts I can slowly nurse RPM's up until it stopes at about 3/4 throttle. Not enough to go on plane. When stopping and shifting to reverse, the shift is difficult and the engine quickly goes to high RPM's. I will drop unit again tonight but i'm wondering if anyone has experienced this. Not so sure I'm getting shift shaft correctly alligned.
thanks

 

[Bob in Tennessee]

My issue is probably how I have installed my cam reverse lock on the shift shaft. Any help is certainly appreciated.

 

[Quicksilver]

 

[faztbullet]

Make sure its the correct gear ratio of 2:1. If you installed a 3 cylinder unit with 1:83 or 1:64 it will kill the top end

 

[Bob in Tennessee]

View attachment 32168

Thanks so much! Now I'm sure I installed it incorrectly!

 

[Bob in Tennessee]

Make sure its the correct gear ratio of 2:1. If you installed a 3 cylinder unit with 1:83 or 1:64 it will kill the top end

Yikes! Never knew there could be different ratios. I bought it used, obviously, so I didn't know it could be different. I'm re-install and see what happens. Is the ratio printed in some type of model number anywhere on the LU?
thanks!

 

[Quicksilver]

You can figure out the GR by turning the driveshaft and watching the prop; put a pair of Vise grips on the driveshaft then put a mark on the gearcase and prop. Turn the driveshaft - it should go around twice and the prop goes once: this is a 2:1 ratio.

 

[Bob in Tennessee]

You can figure out the GR by turning the driveshaft and watching the prop; put a pair of Vise grips on the driveshaft then put a mark on the gearcase and prop. Turn the driveshaft - it should go around twice and the prop goes once: this is a 2:1 ratio.

Thanks! I will.

 

[Bob in Tennessee]

You can figure out the GR by turning the driveshaft and watching the prop; put a pair of Vise grips on the driveshaft then put a mark on the gearcase and prop. Turn the driveshaft - it should go around twice and the prop goes once: this is a 2:1 ratio.

The driveshaft goes around approx 1.75 for 1 revolution of prop. Sounds like I have the wrong lower unit.

 

[Bob in Tennessee]

Make sure its the correct gear ratio of 2:1. If you installed a 3 cylinder unit with 1:83 or 1:64 it will kill the top end

Looks like I have a wrong lower unit. I'm getting about 1.75 turns for every prop turn. Thanks for the learning lessen. Now to figure out what to do.

 

[Bob in Tennessee]

View attachment 32168

Yep! I had it one spline wrong. Thanks so much for posting this. It will go into the Merc file! The crazy thing is, I have owned the engine for ten years and replaced water pump impeller at least twice before. I must have gotten really lucky with the installs. Either that, or I never removed the cam before.