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Runs perfect until

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kbcave

Contributing Member
I just bought a 1987 70hp evinrude on a boat. First time out on it it ran perfect for several miles and most of the day. Then while cruising at speed back towards the boat ramp it seems to have lost a cylinder. I throttled back to an idle. It seemed to be running on all three at idle. With the throttle part way open it would go from 2 to three cylinders back and forth. The finally stayed on all three. Got back on a plane and ran fine. Closer to the boat launch problem back. Would losing one cylinder keep it from planing? Looked at plugs seems to be fine, all three look the same. So coils we're fairly cheap so I put new ones on. Next time out, higher elevation problem is much worse. Still goes between two and three cylinders but stays more with the two? Please help before I start throwing more new parts at it..thanks
 
After more diagnosis I find #2 cylinder is not firing. No spark from coil. Just replaced coils before this tryout. When replacing the coils #2 had a different part number. So it may have been replaced before. What can lead the coil to not spark. Swapped plug wires around and it was still #2 misfiring. Thanks.
 
kbcave said:
After more diagnosis I find #2 cylinder is not firing. No spark from coil. Just replaced coils before this tryout. When replacing the coils #2 had a different part number. So it may have been replaced before. What can lead the coil to not spark. Swapped plug wires around and it was still #2 misfiring. Thanks.
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Bad coil.. can you swap the coils around ? see if it moves with the coil..
 
i have replaced the ignition module and the problem got somewhat better. it still between idle and take off runs alternates between 2-3 cylinders firing. before the new ignition module it would not run on 3 cylinders long enough to take off. now it will take off running on all cylinders and runs really well. when you slow it back down it alternates between 2-3 cylinders firing again..what next?
 
Perhaps do an ohm check on the sensor coils.----Post results of a compression test.-----Does spark jump a gap of 3/8" or more on a test device, yes or no ?-----Have you removed the high speed carburetor jet from the bottom of the carburetor bowls , yes or no ?----Trouble shooting is the path forward.
 
I had a engine once that had the same symptoms.
It turns out that there was a small chip of paint that somehow got behind the float bowl needle valve.
It would float around and eventually block off gas flow. Then get pushed away by the tip of the needle valve when the demand for more gas was reduced because of throttling back.
I naturally thought it was spark related. But nope, it was fule all the time.;)
 
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racerone said:
Perhaps do an ohm check on the sensor coils.----Post results of a compression test.-----Does spark jump a gap of 3/8" or more on a test device, yes or no ?-----Have you removed the high speed carburetor jet from the bottom of the carburetor bowls , yes or no ?----Trouble shooting is the path forward.
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I did an ohm check on the sensor coil, it seemed to be reading bad so I replaced it. I didn't write down the numbers and cant remember them off hand. one set of wires read 0. no help with my problem. took the carburetors apart and everything was clean and flowing freely. Did the compression check and is top 120 middle 120 bottom 110, I tested the ohms on the stator. 50 ohms on the brown and brown/yellow wires. that is an ohm meter with the lowest setting is X10, on the yellow wires I could only get a reading on the 200 setting and it was 2.3..i had put new plugs in on the last tryout and when I pulled them for the compression test the top two were colored black. the bottom plug was still white. with a test plug while the motor was running bottom plug has spark. I didn't remember the 3/8 gap test until I came back to this post..
 
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