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2003 Mercury 90 HP 4 Stroke Only Running on 2 Cylinders

M

MikeyL

New member
Good Morning - I recently bought a 2003 90 HP 4 stroke Mercury outboard. It idles very rough. I can run it at high idle. I changed plugs and checked for spark. When I pull the 2 middle plugs there is no effect to its running. When I pull the top or bottom plug it dies. The fuel filter had a lot of debris in it.

How do I determine if I have a fuel or spark problem?

Thank you in advance for your consideration.
 
You can use a spark tester or similar device, or can test the spark by grounding the spark plug boot with a screw driver, or pulling a plug out with it still in the boot and getting it near a ground while cranking. I would guess that the carbs are heavily gummed up and are unable to feed those 2 cylinders with fuel. Have you pulled the 2 middle spark plugs after running, are they wet or bone dry, any discoloration?
 
Blue_Sky said:
You can use a spark tester or similar device, or can test the spark by grounding the spark plug boot with a screw driver, or pulling a plug out with it still in the boot and getting it near a ground while cranking. I would guess that the carbs are heavily gummed up and are unable to feed those 2 cylinders with fuel. Have you pulled the 2 middle spark plugs after running, are they wet or bone dry, any discoloration?
Click to expand...

Blue Sky - Right you are. I've got spark. So, its on to the carburetors for a rebuild. Thank you!
 
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