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Help with my 402 40HP Mercury

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Hi y'all

I need some help with my 40HP mercury. I believe it is a 1976, it is a 402 40HP.

Ok here is what happened. I was idleing away from the ramp and decide to give it full power, I went about 15 feet when all the sudden it popped out of gear and the engine reved up. The motor did not die but I no longer had foward or reverse gears.

Naturally I thought the lower unit just went tits up on me. I pulled the boat off the lake and went home. I dropped the lower unit to inspect the damage. To my surprize the lower unit gears look just fine. I noticed some water in the lower unit oil and decided to change out the water pump, seals and impeller while I had the lower unit off.

I went in to twin city outboard and took the lower unit gears with me so they could take a look and tell me if anything looked broken. The mechanic told me the cluth dog looked a little worn but he has seen worse. He told me to check my gear cable to makes sure the they are not out of adjustment.

So I get home, put every thing back together. I put the lower unit back in place and hook up the water muffs to check if the motor some how fixed itself. When I fire up the motor I can hear a loud chattering which seems to come from the mid section of the motor. I also notice the motor is not peeing when hooked up to the muffs with water running. This is getting a bit fustrating.

So my question to you is... is it possible the I put the impeller in backwards? Can his be the reason the motor is not peeing?

Second... could there possibly be a problem with the drive shaft teeth which fit into the upper part of the motor and that is the chattering I am hearing.

When I put the motor in gear (forward or reverse) the prop will turn just barely. I am conserned the drive shaft is not turning.

I have adjusted the gear cable and have ordered and new one, although I do not think this is the problem now. Do you think the clutch dog needs to be replaced? When I put the motor in gear I can not turn the prop by hand so that tells me the gear is engaged. I am wondering now if I have a drive shaft problem.

Any suggestions would be a great help. I am in limbo now and not sure where to look next.


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What did the drive shaft splines look like? Take it back off and have a look. Look up inside at the end of the crank shaft at the splines in there. That would also explain no water pumping.

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Hi Bruce,

You are the second person to tell me to take a look at the drive shaft splines so that is what I am going to do. When I had the lower unit off the first time I notice carbon had built up on the drive shaft splines so I cleaned them off a bit. I am going to pull the lower unit off again this morning and take a good look. Have you aver heard of a problem where the splines go bad? Seems like an odd thing to happen.

Thanks for the help. I will let you know what I find.


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Well it looks like you are right. The splines inside the motor look to be rounded and smooth. The splines on the drive shaft look to be in ok shape but I wonder if the drive shaft should be replaced too.

So what to do next. I really want to fix this motor; it has been very good to me the last five years I have owned it. How do I get to the part where the drive shaft connects? Is it possible the get to this part thru the middle part of the motor or do I have to go thru the power head?

I do have a mercury manual for this motor but unfortunantly it does not cover this section. I know that a person can buy chilton manuals online from automobiles; is the a manual that goes into more detail that can be bought online?

This looks to be a complex part to fix and I wonder if it is above my level of fixing. I would like to do the work myself.

Any help would be great.


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The splines are part of the crankshaft. Total powerhead dissassembly. Sorry.

As long as we are on the subject of drive shaft splines, I just want to remind everyone that any time you have the lower unit off the rest of the engine, Its always a good thing to put a little bit of marine grease, they actually make spline grease for this also, on the drive shaft splines.(Not too much so you don't hydro-lock) It is uncommon for this to happen in our part of the country, but obviously, we can have spline failure. It is much worse in salt water conditions.

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Ok I'm not an expert on this but I did have a 70's 18hp Evinrude with the same issue. As I was poor and had no other options, I cleaned up the shaft as much as I could, roughed it up with some emery cloth and JB welded it into the powerhead. I did run the motor for about two more years before the lower unit went. Just a thought...


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