As previously mentioned, backwards running is normally due to overpriming. To solve the problem when it occurs you must run the excess fuel out of the crank case. The easiest way to do this is simply pinch the fuel line shut and wait for the engine to either turn around and run the correct direction or stall. Then start it without further priming and it should run fine. If you can't access the fuel line, then you're stuck with quickly throttling up and back hoping that will turn it around.

Brad