Can anyone help me out with my problem? I have a 400ex bored to a 440 with a 39mm kehin carb when I use the electric start all the motor wants to do is turn over and not fire but when I push start it the motor fires up. it does back fire a couple of times then its alright, runs great.

 

Are you sure the battery is fully charged, If is not it may have enough juice to turn the engine over, but not enough to start the bike. Since the change to the 440 kit, the compression has changed, and the battery may not have enough cranking power to start the bike.

I assume you are push starting the bike after you've cranked it with the electric start. The backfire may be the unburnt fuel from using the electric start. Try push starting it with out using the electric starter to see if the backfire goes away.