1999 Mercury 75 HP ELPTO top cyl not firing

MoJoRisin'

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Aug 2, 2016
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Noticed after a few trips and low top end power that the top cylinder appears to not be firing. Upon inspection of new plugs after a full 3 plug change early this season saw that the top plug is clean and was wet with fuel.

The other 2 plugs have carbon.....also can note that the boat would not get up on plane without weight shifting to the bow and then WOT speed was about 5 mph lower than normal.

Also note that I changed plugs again to make sure the plug was not bad - same problem.

I checked the plug wire for the top cylinder and it showed continuity (basically 0 ohms). Checked the coil primary which was 0.5 ohm and secondary which was 875 ohms. Cylinder below it was 0.4/0.5 on primary and did not check secondary because that one was firing OK.

Do I have a bad coil - or do I need to do further troubleshooting to determine what the problem is??

Thanks in advance!!
Jim
 
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Texasmark

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Dec 20, 2005
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Swap coils, aka CDI modules (I assume that engine has separate modules like my 2002 90 triple). If the problem follows the module you have your smoking gun. I had my 90 for about 10 years and in that time I had 2 modules go bad. One was thermally intermittent. Wouldn't fire initially on the outing then once I got the engine up and running it decided to kick in and remained the rest of the outing, just to repeat the next one. A timing light on the plug wire helps to locate bad firing too.
 

MoJoRisin'

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They are coils (hot and ground connections only). I will switch the top 2 and check to see if lack of spark stays with the suspect coil. I'm going to get a spark checker also to see if it is weak spark or no spark. From what I can tell there is zero combustion as the plug has been spotless and wet after lots of running.
 

Texasmark

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Dec 20, 2005
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I may be out of line here but I doubt it.

My 2002 90 triple had the trigger wires routed through all 3 CDIs in series....daisy chained. One trigger went to all 3 ignition circuits, simultaneously.

The cylinders fire at 120 degree intervals so only one cylinder will be ready to fire (on the compression stroke just before TDC) and create a power stroke at a time. The other two just waste the spark.....no big deal. Buy using one pickup coil for triggers and placing magnets 120 degrees apart in the flywheel, you can put out 3 triggers per revolution, 120 degrees apart and fire all 3 cylinders 3 times with a simple, pretty bullet proof solid state triggering system.

In troubleshooting my CDIs I swapped input connectors between CDIs, and swapped CDI's between cylinders and had no problems so there is no reason in my mind that you couldn't also. Just that you apparently have the Switch Box common triggering enclosure with wires going to all 3 coils and I had my triggers in each module.

Assuming I'm correct, as I never took my flywheel off and looked but I did have the manual and over the 10 years I had the engine I knew the wiring diagrams almost by heart and wouldn't say this if I were worried about the outcome.

There isn't anything to hurt by what I am going to suggest. It's not like getting the cam shaft out of timing on a GM V8 when the nylon capped cam drive gear shears. You might try swapping the "hot leads" (you mentioned) to the coils and see if the problem follows a trigger circuit.

Before running your testing, ensure that you pull the non firing plug and clean it off so that It will be ready to fire. Carb cleaner or brake cleaner aerosols are what I use....just which one is handy.
 

MoJoRisin'

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Checked first by swapping the middle cylinder to the suspect coil and leaving the top cyl plug wire out and spark tested and no spark still so now knew coil was bad.

Got a new one and coil changed and spark tested OK - DONE!!

Thanks for the help!
 
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