The plug is my first thought as well. I have a couple of spares so it will not cost me anything really. I am hoping that the motor will be reliable, as I would like to drop it into another plane I have built and am waiting to fly. I may put it into a GP Corsair or a Cub if I can get it running right. It did seem to improve after a ran it for a few minutes so I am hopeful with a new plug and a bit more tuning work it will really come to life.