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Mercury 3.9hp compression range

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Fishy123

New member
Hello. Trying to diagnose a no start on a 3.9hp outboard. Was getting 60psi off a cold motor compression test. Does anyone know the range this motor is supposed to be at. Manual just says it’s supposed to be within a certain range of the other cylinders, but this is a single cylinder motor so i have nothing to compare it to. I have spark and fuel at the carb. New spark plug, new points and condenser. Rebuilt fuel pump and cleaned the carb and put new needle and seat. Motor will not start just wondering if the compression could be the issue.

Thank you
 
Does spark jump a gap of 5/16" on a test device , yes or no ?-----Should start and run with 60 PSI.-----There is no head gasket that can leak.-----Or look at piston / rings through bypass cover ( 2 screws ) on side of cylinder.
 
I’ll have to check the spark at 5/16 when i get home. It has bright spark on plug set to 0.030. I looked through the bypass yesterday. Some carbon build up on the top. But side looks good no discoloration,some light vertical lines on the side but nothing deep. Rings looked good, nice and bright no discoloration . There was zero play of piston in the bore. If it should start with 60psi im guessing i fudged something up in either the carb or fuel pump. Oh also i did check the reed valves and they were seated properly
 
Fuel pump has nothing to do with motor starting if carburetor bowl is full.-----Test to do.----Remove sparkplug.----Put 7 or 8 drops of fuel in the cylinder.-----Install spark plug.------Try and start motor.----Report exactly what you observe.
 
racerone said:
Fuel pump has nothing to do with motor starting if carburetor bowl is full.-----Test to do.----Remove sparkplug.----Put 7 or 8 drops of fuel in the cylinder.-----Install spark plug.------Try and start motor.----Report exactly what you observe.
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Will do this weekend when I get home. Thank you for the advice. I’ll report back later
 
racerone said:
Fuel pump has nothing to do with motor starting if carburetor bowl is full.-----Test to do.----Remove sparkplug.----Put 7 or 8 drops of fuel in the cylinder.-----Install spark plug.------Try and start motor.----Report exactly what you observe.
Click to expand...
ok I checked the things you recommended.
1. Yes the spark will jump a 5/16 gap.
2. Added 8 drops of fuel directly to piston through spark plug hole. Tried to start it by pulling 5-6 times on pull cord. Motor never tried to start at all, it looked like maybe a small amount of smoke or vapor coming out of exhaust port. Before this test I reinstalled the transfer port cover and reed valves but i did not have the carb on.
 
I ended up taking apart the powerhead to better inspect the piston and cylinder. Found all three stuck rings and pretty bad scoring on the piston from about the bottom ring to the bottom of the piston. Cylinder walls looked fine. Ran my nail around it and never caught and groves or scratches.
 
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