When a model engine starts backwards, it always means that it's over-primed. Close the high-speed needle valve, remove the glow plug, and flip the engine briskly or spin it with the starter for a bit. You want to get the fuel out of the engine. Then replace the plug and open the high-speed needle about 2 to 2-1/2 turns out from closed. Don't prime it.

Open the throttle to about 1/4 to 1/3, apply the starter battery to the glow plug and crank the engine. It should fire right away, and in the correct direction. If it goes backwards, try flipping it backwards to see if it will kick in the correct direction.

Usually, you find this on larger engines and not so much on the smaller ones. Regardless, the effect you're seeing is the result of an over-primed engine. You can sometimes just open the throttle a bit more without having to close the needle and remove the plug and still get the engine cleaned up and running.