When you say the timing is ok, do you mean ignition timing or valve timing? Something is causing a late ignition of the fuel. That's why you hear the explosion in the exhaust. It could be caused from the spark occurring just as the exhaust valve is opening or very hot exhaust is hitting raw gas in the exhaust ports. Are you getting excessive smoke or a raw gas smell? When it is running, do you still get backfire? Sorry I can't be of much help because I don't know anything about the ignition system on that particular bike. For example, I don't know how it gets it's pulse timing.