My '98 YZ250 will occasionally foul a plug within 30 seconds of a cold start, and that plug will never work again. It may start after a good cleaning, but will always foul again right away. Even when jetted correctly, this bike has always been very sensitive to plugs - it will sputter and cut out with an ES, but if I take it out and put in an EG with no other changes it runs perfectly. I bought an Iridium to try, but haven't fouled the EG that has been in there all year, so I haven't tried it yet.