a plane of that size and weight should be a hoot to fly with a black widow so it should still fly well with a babe bee, must be something wrong with that engine. the cox engines are really hard to hurt during break-in so it must be something else. make sure the tank, venturi, and reed are clean and that all gaskets are good and leak free.

if you do get it to run good your next problem will be a very short run time as fastlash said in the post above. if you're on a budget (or just plain cheap) go to a swap meet and find yourself one of the product engines that cox sells with their RTF toys. the newer "sure start" engines are great runners. these engines don't have an integral tank so you can use any size tank you want (within reason).

dave