low compression 50hp merc

nhvegas · #1 (**permalink**) 06-28-2006, 08:59 PM

I have a 2000 50 hp merc bigfoot not efi. There are two cylinders with low compression. Cylinder 2 has 30lbs and cyl 3 has 90 lbs. Is this simply a head gasket or is there something else causing this. It appears that the head had already been replaced once on this motor. Any help would be appreciated. Thanks

boats.net · #2 (**permalink**) 07-05-2006, 08:21 PM

It could be either, the head is not that hard to remove to see what the problem is.

deep_blue · #3 (**permalink**) 07-07-2006, 03:44 AM

One thing I have tried is to put the suspected cylinders at TDC with both valves closed and use a blowgun with a wide rubber tip to tightly plug it in the spark plug's hole. Continually fill the cylinder with high pressure air and listen or feel for air escaping.

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1. Open the oil-fill cap and feel for air (if you feel a lot then its bad rings or cylinder walls).
2. Put your ear up to the carburetor inlet with throttle wide open and listen for air (if you hear large amounts escaping then its the intake valve)
3. Listen to exhaust outlet (if you hear large amounts escaping then it means the exhaust valve)
4. Put your ear right up to the cooling water inlet and outlets (depending on the engine's design it could mean bad head gasket)
5. And finally take a cup of soapy water and slowly pour it all around the seem ware the head and cylinders come together, if you see bubbles then that also would indicate bad head gasket.

Hope that helps

deep_blue · #4 (**permalink**) 07-07-2006, 04:37 AM

sorry i missed one, because the two problem cylinders are right next to each other remove both spark plugs, compress the cylinder thats at TDC with air and listen to the other cylinder. if you hear or feel air between the two its the head gasket (and because its two cylinders right next to each other this i am willing to bet is the problem) [img]smile.gif[/img]

a much simpiler approach is to put the outboard in gear, shine a small flash light (at night) down inside the cylinder and move the piston by spinning the prop with your foot. it will let you see the condition of the cylinder walls or look for dents on the crown of the piston incase part of the valves has cracked off. if neither existist then you can almost bank on the fact that its just a stupid head gasket (and i dont know your experience level but if you do the work your self make sure you follow the torquing pattern proceidure and invest in a nice break-over torque wrench! usuily $25)