Abnormal plug wear?

[ktoelke54]

The motor is a late 90’s Mercury 25hp 2 stroke. I’ve owned it three years and changed the plugs once, about 40 hours ago, best guess. It started to idle a little rough and take a little more effort to start, so I bought some new plugs. Upon removing the old plugs, I noticed that the plug from the top jug, the center post electrode was worn completely off, just gone? The bottom cylinder showed normal wear, the gap being just little wide. Other than the above mentioned issues, the motor seems to be running fine. Any ideas what could do that to a plug?


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[GYPSY400]

Is this motor one carb, or two? Possibly leaned out on the top cylinder?

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[ktoelke54]

Is this motor one carb, or two? Possibly leaned out on the top cylinder?

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Single carb


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[Pappy]

Abnormal plug wear can be as simple as a spark plug with a higher than normal internal resistance or a plug wire/boot combination with a high resistance or not fully plugged into the coil .
In both situations the ignition system is forced to work harder to overcome the increase in resistance. When it works harder the result is a much higher than normal voltage passing across the plug gap which will wear the plug out faster.
This may not be your issue but it is a fairly common one.

 

[GYPSY400]

Abnormal plug wear can be as simple as a spark plug with a higher than normal internal resistance or a plug wire/boot combination with a high resistance or not fully plugged into the coil .
In both situations the ignition system is forced to work harder to overcome the increase in resistance. When it works harder the result is a much higher than normal voltage passing across the plug gap which will wear the plug out faster.
This may not be your issue but it is a fairly common one.

Could this happen if you use 5Kohm plug boots on a system that didn't have them originally?

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[Pappy]

Remember electricity will always take the path of least resistance. It is lazy! Or smart....take your pick.
Modern ignition coils can easily make 30-40,000 volts however at idle they may only need to make 4-7000 volts to jump a normal gap. More voltage needed as dynamic compression builds when throttle and load are increased on the engine. Coils compensate for this very easily.
When you add a resistor cap into the ignition system the system will react by working harder to overcome the resistance.