Reversing Wires for Outboard Spark Plugs

2/22/16 @ 11:50 AM
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Mr.Bass
Mr.Bass
USER since 5/18/02
I have a new 20HP Mercury 4-cycle. Decided to practice for the first time on changing plugs (and the prop). I failed to mark the plug wires. A phone call and an all too common memory failure followed. It is possible to reattach any one. But one wire was a bit shorter than the other, and seemed more difficult to put onto one plug. So I took the approach of their proximity and what was the easiest way to re-attach them. Manual and internet not much help. One friend says the motor will not start if plugs are reversed; another says it will back-fire. One Internet report is no damage can occur if the motor is only started for a few seconds. So I will wait until lakes open up and take it out for a first time ride. Any thoughts/suggestions would be appreciated.
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Displaying 1 to 6 of 6 Posts
2/23/16 @ 9:36 PM
Mr.Bass
Mr.Bass
USER since 5/18/02
Thanks for your comments. I have contacted both MW marine and Mercury Marine main service. All of what you have said has been verified. First off, I have found that I did replace them correctly. One of the cables was a bit longer than the other. In order to put them on incorrectly, the user would really have to stretch and tug on the short one. I really didn't notice this, and was just lucky I did put them back on correctly.

MW stated that the 15 hp and 20 hp have basically the same setup regarding plugs. So I looked inside one new model. It was hooked up exactly as I had done. Took a photo of the setup, just in case of another lapse in memory.

Now all I need is 'ice out', some great spring weather, and a chance at getting on the water. Thanks again.

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2/23/16 @ 5:28 PM
Plmlk
Plmlk
MEMBER since 12/18/07
Thank you nif...

Mr. B - The more I think about it the more I believe that If you have the plugs reversed your spark will be 180 degrees off, meaning when your at TDC with one cylinder the spark will be present at the other piston which will be at the bottom of the cylinder. It'll likely do nothing more than backfire, but that certainly shouldn't cause any damage.

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2/22/16 @ 10:37 PM
nihsif
nihsif
MEMBER since 6/15/01
Plmlk makes sense as usual

if this is a new motor, or even if not, you should be able to call or email dealer and get your answer there...small effort, big gain

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2/22/16 @ 7:47 PM
Plmlk
Plmlk
MEMBER since 12/18/07
Take with a grain of salt, but here are my thoughts....

The plug wires get used to being in a certain position for a long time, and you should be able to see which way they want to naturally lay.

You noted it was difficult to connect the shorter plug wire to the furthest cyclinder - that is likely your answer that it is bass ackwards...

Also, off the cuff, if you had the wires reversed you should be 180 degrees off, meaning that all it should do is back fire, or run like absolute crap.

Put the wires where you think they belong, and if the motor starts easily and runs smooth, there is nothing left to check. If the motor backfires, you have them reversed.

For a short run period, I truly can't imagine there being any damage.

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2/22/16 @ 12:21 PM
IEATFISH
IEATFISH
USER since 2/21/09
I would confirm locations before starting being a new motor.

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