How two stop back firing on a two stroke motor bike?

There are several things that can cause back fire or "engine spit." The first would be to put in fresh fuel because there could be water in it or it could just be old. If that doesn't work, I would suggest making sure your carbs are clean because this would cause an insufficient source of fuel needed to run the engine. Another thing would be to make sure you don't have a bad ignition coil. This could cause firing at the incorrect times. If none of these work, you are possibly looking at a stuck valve, or a sheared flywheel key.