Mercury 115 4 cyl first steps?

I just purchased a great boat and trailer with a 'bad' Mercury 115 4 cylinder (1991).

(I have a Mercury 115 6 cyl to put on my new boat from my old boat.)

The previous owner was told by the marina that the old 115 4 cyl motor has a low cylinder. The prevous owner said it would start and idle great but would not come up to full power. What checks should I do before I remove this motor from the boat? I would like to try my hand at repairing it this winter. I do have a Seloc manual for it.

Re: Mercury 115 4 cyl first steps?

I wonder if a compression test is the first step. Since there isn't a head or valves a leak down test wouldn't show much, right?

Re: Mercury 115 4 cyl first steps?

Compression first and all should be within 10psi of each other, leak down test is best but you need to fab a piston stop...

Re: Mercury 115 4 cyl first steps?

Throw away that seloc manual and buy a merc manual. The merc manuals are far better.

Re: Mercury 115 4 cyl first steps?

Got a leak down tester and compression tester and was all ready to start.

Apparently the FIRST step should be to take out the spark plugs and turn the engine over by hand a couple of times. The engine would only turn over about 270 deg. Putting my fingers over each spark plug hole I found that pistons 1,2,3 (starting from top) all had good compression. Bottom one appeared to be the one not going to TDC and also had NO compression (or suction) when turning flywheel.

I checked the water pump and the components and impeller appear nearly new.

Any advice whether I should tear it down and see what is, parting it out, or ebaying it broken?

Thanks in advance!

Re: Mercury 115 4 cyl first steps?

Not going to TDC? So the piston is not moving when you turn the flywheel? Sounds like it threw a rod.