The plugs will also die if your a bit rich on the main jets.
On my 18 I went up on the mains jet sizes after doing the airbox mod.It was stock (128's) so I went up to 135's but it killed the top plug within about a mile of riding.I went down to 132's and it still did the same but I was able to ride it for a lot longer before it died again.I went back to the stock sizes and have never had anymore problems.
_________________
Harley Davidson is the worlds most efficient way to turn petrol into noise without the harmful side effect of horsepower.