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one plug won't fire

E

entsminger

New member
Missing on one cylinder..... I have a Mercury early 80's 7.5 outboard motor Serial # 2804295 which will not fire on one spark plug. I have switched the coils and it will not fire on the same side so I don't think it's the coil. I've checked the wires and connections and plugs and see no problem there. I've loosed all connections except those under the fly wheel and cleaned them and tightened them all. What is the next most likely cause of a plug not firing?
I took it to the shop twice and they said they fixed it but a couple of miles down river the one plug would quit, twice . Sometimes the plug would activate and it would run fine for a while and other times it would stop. Now it's dead. Is there anything under the fly wheel that may be faulty?Any suggestions what to try next?
There is this black sealed box like thing that the wires all converge in? Could that be bad and what is that??

Thanks
Scott Springston [email protected]
 
It is a switch box or Ignition module , or CDI box.-----------Pricey unit. -----------NLA from factory , but available from aftermarket.
 
It could be stator or trigger, both can be checked with a digital multi meter, and the reading you would be looking for would be in your manual. If they both check out OK, then its probably your switchbox.
 
That # makes it 1970 not early '80's.What colour is the band? Red would match 1970
75 (7,5) 2529895 - 2810636 1969 2 cyl
75 (7,5) 2810637 - 3016580 1970 2 cyl
75 (7,5) 3016581 - 3274632 1971 2 cyl, Compliments of TMD.
 
Not sure about this motor but does it have wires coming of the coils? try swapping them side to side. I know on cars they sometimes build up too much resistance which can cause intermittent no spark
 
remove all plug wires, measure the resistance of the plug cap to ground and compare readings of all cylinders. If you find a reading that is hard to get or intermittant, you have a broken plug wire and it will melt down your coil. I had one with a broken wire 1" from the end of the plug that read intermittant and my coil blew. Measure the input of the coil to ground and compare all readings. Check grounds on all coils. No expert, but if all that checks out, you're probably into a switchbox or a trigger.
 
I've eliminated the coils or the wires and the plugs as the problem. I switched the coils from one side to the other and one side works and one side doesn't. Where is the trigger? Under the fly wheel? I'm guessing this engine has no points? I have no repair manual for this engine. My manual only has typical maintenance.

Scott
 
It has one trigger coil ( proven to be working too )---No points !!-------------Check the charge coils --------------You likely need a CDI / switch box
 
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