1986 merc 40hp 4cyl

[Aquaman0326]

I recently purchased a new-to-me duck boat with a 1986 mercury 40hp tiller. When I went to look at it I discovered it has low compression on the 4th cylinder. It started a little rough, but ran fine on the hose before I found out about the compression issue. I’ve never done a power head rebuild, but I’ve got a spare motor to throw on it for now so I figured why not. Does anyone have any good places for resources on where to start? I’m fairly certain the carb is the culprit that caused the wear because the spark plug in that cylinder is still brand new. Not even black on the tip. Seems like a decent motor and wouldn’t mind spending the time to rebuild but I can’t seem to find a whole lot of information on the 4 cylinder models of this motor. Thanks in advance!

 

[Faztbullet]

Bottom crank seal bad and cylinder sucking water..this is common problem on Inline motors. Needs to come apart.

 

[jimmbo]

You are aware that, your Engine does not have a removable Cylinder Head?
Pop the Transfer Port Cover off and check to see if the Rings have tension, and to see if the Piston Skirt is scored. If you have a Fiber Optic Camera, stick it in the Spark Plug Hole and take a Look at the Cylinder Walls.
Your engine has 2 carbs, each feeding 2 cylinders so unless #3 also has a Plug that also looks New, I doubt the Carb is the Problem.

 

[Aquaman0326]

You are aware that, your Engine does not have a removable Cylinder Head?
Pop the Transfer Port Cover off and check to see if the Rings have tension, and to see if the Piston Skirt is scored. If you have a Fiber Optic Camera, stick it in the Spark Plug Hole and take a Look at the Cylinder Walls.
Your engine has 2 carbs, each feeding 2 cylinders so unless #3 also has a Plug that also looks New, I doubt the Carb is the Problem.

I didn’t know that! I was confused how #4 could top out at 50psi and the other three are a perfect 120. If the motor doesn’t have a removable head then I guess you have to pull the entire power head off to get to the pistons? I’m going to get a camera from Amazon and see what that cylinder wall looks like.

 

[racerone]

Total tear down is needed.

 

[Aquaman0326]

Let me clarify, I know im going to need a rebuild. I just can’t seem to find any resources that will tell me where to start? YouTube and Google turned up nothing, I can’t even seem to find a kit for it because all the 40hp 4cylinder kits start at year 1990.

 

[jimmbo]

That Motor has been around since the late 50s. For many Years it was known as the 50hp. In the mid 80s, Mercury Prop Rated the Engine, as a 45hp, then a 40hp.

Get a Factory Service Manual for that Engine as it will have the Info you will need to rebuild and readjust the Linkages after rebuilding.
A bit of effort will be needed to find the correct Manual, as the Engine's Serial # needs to be covered by the Manual

 

[racerone]

Lots of new /used parts available.----As stated marketed for many years.