A run away 2 stroke usually means it's sucking air somewhere.
Check for bad gaskets on the carb, or pin holes in the fuel line.
Funny I should see this post tonight. I happen to have a chainsaw in my shop right now with this exact issue. The problem with it was the throttle plate had worked loose on the throttle shaft. It wore oblong, and had the throttle bound in an open position...
Rick
October 2005
howtomendit.com for free repair help, information and advice.
