Spark failure 1997 J55RSLC

[Platypus E65]

Recently while running the motor WOT it started to run on one cylinder, checking to see which spark plug failed I found that the top spark plug had broken in half around the insulator. Installed a new plug, started up the motor fine, planed the boat up to WOT and the plug started misfiring after a minute or so. This time I found a tear on the cap above the terminal of the plug. Taped it with electrical tape, once again the motor started up fine and on the way home the plug started misfiring again after a minute at WOT. At home I switched the spark plug wire and caps with a used one with no difference, starts up fine, revs up to WOT then dies after a minute of running.
What would be causing the plug to misfire or die after a minute of running fine?
How do I diagnose this?
TIA

 

[tashasdaddy]

Re: Spark failure 1997 J55RSLC

bad coil. swap it with one of the other coils on the motor, hopefully the problem will follow the coil. if it does, replace coil.

 

[Platypus E65]

Re: Spark failure 1997 J55RSLC

Thanks, tashasdaddy. AFAIK there's only one coil on the 55, unless I totally missed the other one.
I'll look for a good used one to try when the weather co-operates.

 

[Platypus E65]

Re: Spark failure 1997 J55RSLC

I ordered a coil from iboats and finally got a chance to try the motor the other day. It works fine. Thanks again.