You shouldn't be fouling plugs, but that could be a lot of things. 2-stroke MX engines shouldn't really idle much. Your bike could be fouling plugs because it's jetted improperly. It could also be fouling plugs because of yoru riding style. 125's don't like to be lugged around at low speeds in a high gear. They don't like to be crused around at low speeds for extended periods of time. I like a few seconds of idle on my 2-strokes. This will give the best throttle response, provide some (but still minimal) engine braking going into turns, and make the bike less likely to foul plugs when running at low speeds.