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1994 Mercury 25hp missing on top cylinder till warm up.

S

Ssimmons

New member
What causes the 25 hp to not fire on the top cylinder till it's warm? If I start the engine cold it won't fire then after about 3-5 minutes it will start spitting for about 30 seconds and go to firing fine. It will then start and run fine as long as the engine is warm but as soon as it sets for a day or two it does the same thing again. How do I go about figuring this out? I hate to just throw parts at it not know how to really fix it. I'm thinking it's the switch box but not sure.
 
sounds like its a fouled spark plug. compression check the cylinders see if number one has less than the other. when its cold it fouls instantly, but after its been running and engine warms up the cylinder dries out and spark plug is able to fire again...once the engine is warm your fine cause the piston has expanded and not causing any leak. also could try a new plug in that cylinder before you start the cold engine. see if it fires. if not firing then something electrical. electrical wouldn't matter if the engine is cold or warm, just the vibration of engine would fix the short in an electrical issue.
 
Mark Warner said:
sounds like its a fouled spark plug. compression check the cylinders see if number one has less than the other. when its cold it fouls instantly, but after its been running and engine warms up the cylinder dries out and spark plug is able to fire again...once the engine is warm your fine cause the piston has expanded and not causing any leak. also could try a new plug in that cylinder before you start the cold engine. see if it fires. if not firing then something electrical. electrical wouldn't matter if the engine is cold or warm, just the vibration of engine would fix the short in an electrical issue.
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I has new plugs but I will do a compression test first then change them again if it checks out. With that said I did pull the wire on that cylinder and it wasn't firing when it was missing that cylinder.
 
racerone said:
Switch the coils and see what happens.
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Okay so I switched coils and its running like is suppose to now so i'm going to get a set of coils and wires sense they were pretty corroded and one wire had a broken tab where it plugs into the coil at. Also found the black and yellow wire from the aux stator coil that powers the switch box had been rubbing the magnet on the flywheel and was almost wore through, so i fixed that also.
 
Not sure if that year motor used individually attached wires. If so, do yourself a favor and make a wiring diagram BEFORE you remove any wires.

Jeff
 
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