I have a ultra mbx pro, and had all the same problems.I very carefully rewraped the string after winding the recoil spring,watch your eyes.Using the one way berring on the bak of the crank to be sure the string is wraped clockwise around the spool.I have done that so many time,and also just spent I think 21 dollars or so for a new one it was crazy.I'm not sure what exactly kind of set up you heve as far as the fly wheele on the bottem of your car but my advise save your self a world of agrivation and invest in a starter box.I did.hope that helped.[sm=thumbup.gif][sm=thumbup.gif]