MOTO-4 Eats Spark Plugs

[theHoofer]

I have a 1986 YFM-225S that runs great with a new NGK-DPR-7ES spark plug. But only for about 30-40 miles, and then it gets harder and harder to start, and eventually won't start. Then I put in a new plug, and it's like a new bike again.

Any idea why? It appears to be getting carboned up, but eventually, even if I clean up the plug, it won't spark.

What else should I check/replace?

Frank