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1979 55hp evinrude misfires

J

jamesan

New member
New here, but glad to see a forum that is actually active. I have a '79 55hp evinrude that has an occassional misfire. By occassional I mean that when the throttle is advanced enough to give the small boat some speed the motor starts running very smoothly EXCEPT about every 5-20 seconds it misfires 1 time. The engine sort of goes "BAM", and then it runs perfect for another 5-20 seconds before misfiring again. It goes on constantly regardless if the engine is cold or warm. It certainly seems like a spark problem (replaced the plugs already) that only shows up under higher rpm's. The heavier the load, or the higher the rpm's the more frequent the misfire.

From the manual I have learned that it is a Type II Capacitor Discharge Ignition. I do not know exactly how to troubleshoot this problem as it is difficult to simulate the high rpm's at home.

Any thoughts?
 
Yes, that rare misses apparent under load only are difficult to find. You need to troublehoot on the water.
To distinguish between a fuel and sparking issue I suggest to "listen" to the sparking with an AM receiver. Take a cheap AM radio and tune it between two stations. Connect a headphone to the radio and place it somewhere near the engine where you can hear the sparking of both cylinders.
A single miss might be difficult to hear for a non-trained person, but a "BAM" is probably a miss on both cylinders and thus easier to identify.
 
Put a timing light on one cylinder at a time.--------Take the boat out and wind it up------------An ignition miss will cause the light to stop flashing.---------------Could also be a problem in the lower unit , what does the oil look like ???
 
Interesting thoughts. Thanks for the input. I might try those ideas.
The lower gear oil seems fine except for the water that gets in it. I do not think that the lower unit is binding or anything, if it was I probably would have destroyed it by now.

Keep the input coming, I appreciate it.
 
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