1975 Mercury 200 20hp health question

[pbeamtn]

I just bought a small boat that came with a 1975 Mercury 200 20hp, serial number 4148487. The previous owner said the carb had been cleaned and the impeller replaced last year, but it was hard to start when cold. Once started, it would start easily. I think it sat for longer than he remembered, because I had to clean the carb and replace the impeller, and I'm finally to the part where I can get it to run, but it is very hard to start initially. In fact, to get it running after I rebuilt the carb I had to spray a little starting fluid into one cylinder and then it fired and ran on the very next pull. After getting to this point, I measure each cylinder at 60psi cold when pulling a couple of times with the manual starter. My understanding is that it is good they are the same, but it is a rather low number. I have also noticed that when it is running and pumping water, there is a small leak from the power head, perhaps from a joint in the block.

So, my big question is, how do I determine the overall health of this motor so I can decide whether it is worth putting more time and money into it? I have seen some comments that motors this vintage often need crank seals, but how do I know if I need that, and will that in any way make the motor easier to start?

 

[cwphoto]

Hello:

I am in a similar situation as you, and have mostly just been reading. I do suggest you pick up the manual, as there's a lot of great information to be had there. My understanding was brand-new this engine produced 105 psi compression, so 60 psi sounds like it's been well used, but not dead. Mine was producing 90 PSI, and it fires right up. I have the electric start version with automatic choke. Speaking of chokes, have you confirmed yours is working correctly? Are you pumping up the primer bulb until it's hard? I've heard enough issues with the fuel pump diaphragm that I would also suggest changing that out, as that is fairly inexpensive. I can't help you with the crankshaft seals. At least one other person recommended seafoam to help bring back some compression. I can say this motor will push a small 14 foot jon boat over 30 miles an hour, which on such a small craft is almost scary! Even if you are getting 60% capacity, you could still have a blast!. A new 20 hp motor is what $3000, and once you start getting into electric start and remote steering, add another 500 on top of that. So, to me if I put $300 into this thing, I think I've got my monies worth, and then you could turn around and sell it for parts if and when it dies. A lower unit by itself goes for a few hundred dollars. Anyway, I am certainly no expert on these motors, so will be watching to hear what the gurus have to say.

 

[pbeamtn]

Here is what I have concluded after doing research and referring to a general Mercury repair manual: There is no way I am going to be happy with 60 psi. The manual I have says 110 psi. Electric start makes a low compression engine usable, but I'm not going to generate enough speed by hand to overcome the losses. I am also concluded this is the weirdest motor I have ever seen as I puled off what appears to be the head only uo to find out it is just a cover on the waer jacket around the cylinders! To inspect the cylinder walls would require splitting the block in half, which I'm not excited to do and time is passing quickly.

So, I found a powerheasd on eBay that claims 115 psi and is $80, so I have purchased that and will put my effort there. One set of rings is $60, so I figure another powerhead is worth the risk. I will go ahead and replace the crank seals since it should be relatively easy with the powerhead out. I'm uncertain whether to replace any other gaskets while I am at it. Anyone with experience on what to check out on a new used powerhead feel free to chime in.

 

[pbeamtn]

I'm going to answer my own question, and hope this helps someone else.

First of all, I used a Harbor Freight compression tester, which measured 60 PSI on each cylinder, which led me to believe my pistons were worn. So, I purchased another powerhead on eBay and decided to work with it. The seller said the compression was 115 on each cylinder, but after assembling with the new powerhead and measuring the compression, I still get 60 PSI. I have measured 4 cylinders and all measure 60 PSI. This is suspicious. A little research indicates 60 PSI is a common reading on HF compression gauges. My hole reason for doing the swap is now suspicious.

One thing i did learn while doing the swap is if you pull the flywheel you can check the top crankshaft seal. Mine had a puddle of oil, so it is leaking. This could have been the reason for my hard starting. I'm not sure how easy it is to replace, but it is relatively easy to check.

 

[racerone]

Remove the tranfer port cover and inspect pistons and rings.

 

[pbeamtn]

I removed the cover (the bolts were kind of hard remove due to thread locker or corrosion, not sure which) and looked at what I could see. I'm not sure exactly what to look for, but things are different than I expected. The 3 rings on each piston exist and appear to not be broken. They all flex a small amount when pressed, but the ring closest to the crank appears to have less spring than the other two. The cylinder walls I can see are not scratched, but they appear to have some kind of brown film on them. Moving the piston back and forth does not seem to disturb the film. Both pistons below the rings appear to have the same film as well and it does not wipe off. The pistons below the rings also have some discoloration and scratches in them. I hope these pictures tell the story.