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1987 mercury 25 hp compression

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ford1286

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I have a 87 25 hp mercury outboard. Its been running ok but ive decided to fix up the boat during winter. I did a compression test on the motor and i have exactly 75 psi on both cylinders. I then decided to do a wet compression test and came up with the same thing. I tested the gauge with my air compressor and its pretty close to the psi in the tank. Any ideas on whats causing this?
 
That's what I was thinking also but compression should have improved with the wet test. I also noticed the exhaust box was leaking would that cause compression loss?
 
There are no gaskets / valves involved with compression loss on that motor.------You can remove some covers on the side of the block to inspect pistons and rings.----3 screws each if I recall.----If there is no damage to pistons / rings then just run this motor !!
 
I tore it all the way down since I've found out they make powerhead rebuild kits. Found some scoring on the lower piston but nothing up top which still makes me wonder with the equal compression
 
What is your serial #? You can't bore the 1987 block, it is a chrome bore, does your block have a thermostat? There are 2 blocks made in 1987 the design 1 block has no thermostat and almost no parts availability. The design 2 block has a thermostat and has pistons available for it and individual parts. No one offers a rebuild kit for the chrome bore block that I am aware of. The only place that you can get pistons for your motor is from Mercury, nobody else makes them. They make rebuild kits for the mercosil block which is a later block came out in the early 90s. You can re-sleeve a chrome block and you must use Wiseco pistons if you do that.
 
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OB164113 is the serial. What are my best options? A 5k new motor isn't one of them lol. I've seen blocks that are already bored and re chromed and also I guess I can bore it and have it sent out to be chromed by us chrome. I'm open to all ideas at this point
 
Why do you think the chrome is bad? Very seldom does the chrome wear out. Use muriatic acid to clean the chrome do not get the acid on the aluminum. As long as the chrome is not flaking off your good to go. DO NOT BORE THE BLOCK!!!!! If you choose to have it re-chromed send it in and whoever does the chrome will take care of everything. There is no oversize piston for your motor so if you bore it you can throw it away. The only oversize pistons that are available for your motor are from Wiseco. Wiseco pistons require a cast iron sleeve to be installed as they do not make a piston for the chrome bore. Use a long Q-Tip with acid on it to to remove the aluminum from the bore. Then take a scotch brite pad and clean the chrome off so it looks pretty. You may only need a couple of pistons.
 
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Are you sure that is not aluminum transfer stuck to the chrome. Try muriatic acid on a Q tip. If it is aluminum the acid will bubble and will dissolve off of the chrome.
 
Yes just use a Q tip and a little acid. If the chrome is actually damaged you will know but it takes a lot to wreck chrome.or take the block to a dealer and see what they say about it.
 
My buddy owns a machine shop and he has it now to look at it for me I'll let him know what you said and see where to go from there. Thanks so much for your help I'm lost with 2 strokes lol
 
There is definitely a groove in the cylinder wall. Question is if I throw new piston and rings in it will it be ok or should I try to find a new block?
 
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