A lot of two strokes need to be opened every few minutes or so to keep the spark plug burned off.  If you go 30 minutes or so without opening it up, it may foul the spark plug when using the recommended heat range.

Installing a heat range one step hotter would probably cure the spark plug fouling problem if you are going to go 30 minutes without opening it up.  There is a danger of causing pre-ignition when using a hotter spark plug when you open it up for too many seconds at a time especially if the engine is up to normal operating temperature.

Also set your spark plug gap to .018" to .020".