I have a STihl 028 AV Super chainsaw that will run for a couple of minutes at full throttle. When I stop using it and let it idle it quits. I have put fresh fuel into it, cleaned the sparkplug, cleaned the air filter. What else should I do? I have taken the spark plug out and cleaned it several times getting a severe carbon buildup. Can anyone suggest to me a particular way to go with this? I have crews needing it up here in Alaska and they ar dependent on it.