Fuel and fire are all that is required to fire the motor up; gotta have both. New carb?? that would be the first place to look. BTW, what model bike and what other modifications, if any, exist? New coil, wires and plugs? if fuel is there, that's the next place I would look. I have seen new coils bad out of the box; ya never know.
Stick to the basic trouble shooting techniques and you will figure it out.