I have retired all my 2 stks and now use all 4stks I now have 10 of them and love every one of them. Just be sure the valves aen't too tight.I had much trouble at first with the smaller one when I set the valves at the bottom of the tolerance window and the exaust valve got hot enough too cause it to nold open and kill the engine. It ran fine on the ground but as sone as I tried to take off it would warm up and die.Set the exaust valve to 009 and it runs like a champ.