Greg, ya beat me to it mate. Flypast, it is probably over compressed as Greggles stated. I have a Russian MARZ 2.5 cc engine that bounces back and forth as well when it is at the over compressed, 1/4 turn out on the compression screw will alleviate that. I find that when my PAW engines are flooded, they do fire but whack the chicken stick with a vicious bite.