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2 dark plugs 1 shiny clean

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brian1970

New member
I am having some problems with my 1988 75 horse Mariner. Just got it back from service. Carbs were cleaned new fuel pump and fuel line replaced. The motor wont idle and wont run at full power. After it shuts off it wont start back up. When I pulled the plugs I noticed that #2 was clean and shiny. Compression is good and there is spark on all three cylinders. What could be going on here?
 
No on water jacket if you are talking about over the top of the pistons. Merc uses a cast integral head so there is nothing at the top of the cylinder but the spark plug hole. No detachable head. like with OMC. Water intrusion would have to come from some other area. Exhaust cover/water jacket cover possibly or a hole in a cylinder caused by corrosion. I had that happen to an engine years ago. If you operate in salt water that is surely an option. But guarantee you that's your problem.

Mark
 
Does spark jump a gap of 7/16" on all 3 cylinders ?----------Have the reed valves been checked on that cylinder ?
 
brian1970 said:
What then would be my next course of action to fix the problem?
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Stay focused. You don't want to be running the engine with water in the cylinder regardless. My starting points would be to remove the covers over the cylinder and on the exhaust manifold cover and mid plate. Have a good look around. If you see a lot of corrosion caked up you can bet that you have block penetration. To test the cylinder you might make a compressed air adapter and insert it in the spark plug hole. Put maybe 10 psig on working the piston up and down the cylinder (pull the other two plugs) and just pull it through with a crank rope, and with a squirt bottle of soapy water, squirt around the cylinder and look for bubbles. Obviously if you have any you are ready for a new block. I don't know what a professional mechanic at a dealership would do.

Mark
 
Check the compression on all 3 cylinders...a leak will stick out like a sore thumb. If compression is down on #2 verify it is a water leak by removing the plugs, kill the ignition w/the lanyard switch and attach the muffs and hose. Crank it to see if water comes out of #2. You may need to place a wad of paper towel inthe plug hole to see the water.
 
Also if its a water in cylinder problem the plug will have water droplets on it when removed from cylinder and will rust around electrode if let set overnight.
 
I just pulled the #2 carb on my motor because I couldnt get any power out of my #2 cylinder. After alot of cranking I was able to get it going. I pulled the wire off of #2 plug and there was absolutely no change in how it ran. I know I have spark and compression on that cylinder. The reed valves are good and the carb is clean. I am totally dumfounded at this point. What else could it be?
 
Fuel or spark. A timing light with the engine running is a quick way to check spark. Sensor only has to be like an inch from the wire to light if the CDI and plug are working.

An aerosol can of Sea Foam into the throat of #2 with the butterflies open should light it off if fuel. Sea Foam contains oil unlike a lot of starting fluids.

A little water in the cylinder will not stop it from firing and producing power but you need to find out where it is coming from and get it stopped.

Mark
 
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