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70 HP Mercury Lower Unit

T

thebubba

Regular Contributor
You all recently helped me out with choosing the correct prop size. Finally got it honed in. I have a more serious problem with the motor. The lower unit no longer goes into reverse. and is leaking a good amount of oil from around the propeller. This boat is new to me so the first thing I did was replace the impeller and gaskets around it. I also changed the lower unit oil. The old was very low and had a little water in it. Going into forward I have to put it forward a little ways before it engages, and it clunks into gear instead of kind of rattle then engage like others I have used. This is a 1978 70 hp mercury. I repair small engines, so I am wondering how big a job this is to repair, or should i find another lower unit and what other models are interchangeable with this model. Thanks for the help in advance.
 
Do you have a manual...you need one. They sell Seloc here. Check eBay for a factory manual. When you changed the impeller did it slip into another shift position? Can you see oil coming from the prop seal? You can drain the oil and pressure test the LU to find the leak point.
 
It is possible it moved into another shift position while I changed the impeller. I have it removed again. I can feel neutral very clearly, the propeller spins freely either direction. If I shift one direction when I move it far enough it engages into forward (I assume). If I move it the other direction (reverse) it tries to grab something but just clicks when I rotate the propeller either direction, no matter how far I rotate that direction, it never grabs anything.
 
Just found a 1974 50hp model 500 complete for $250. Took the lower unit off and they are an identical match. The gear system on my old unit is definitely shot as the new one manually shifts into gear much easier and smoothly. Hopefully it doesn't have any leaks.
 
Good call, I suppose I can rotate the shaft on both and count the rotations to see if they are the same. I won't be able to test the reverse on the old one.
 
So i checked the gear ratios the best I could. It appears my original lower unit was approxiamately 1.64 shaft rotations to 1 prop rotation. My new unit off the model 500 is 2 shaft rotations to 1 prop rotation. So the new one is lower geared meaning it will rotate the prop fewer rotations. This means my engine will over rev unless I can put a big enough prop on it, but then I am changing everything on the design of this engine.
 
I took the boat out today and it did okay, but didn't want to start up too badly, even though it did fine in the driveway. Finally got it running. I had to rev it up in neutral to get it to not bog down and die in reverse backing off the trailer. It didn't want to idle down this trip very well. I had to keep the idle a little higher so it wouldn't die when I put it in gear. It died when I would try to reverse out in low speed. I tried a different prop and it felt a different. I have two props, which will give me the lowest rpms? I have a 12" 9 pitch or a 10 and 3/8" 15 Pitch.
 
I have two props, which will give me the lowest rpms? I have a 12" 9 pitch or a 10 and 3/8" 15 Pitch.
Click to expand...
10 and 3/8" 15 Pitch should drop the RPM. Turn the carb isdle mixture screws CCW 1/2 turn to slightly richen the mixture.
 
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