It could be anything from water in the gas, dirty fuel filter, vacuum leak, coil wire, igniter, coil, pickup coil, or a squirrel stuck in your exhaust pipe.
Need more info. Is it running on both cylinders or only one? Have you checked the wires to the plugs. Is there spark at the plugs? Have you run any Seafoam through the fuel tank to clean the carbs? Have you performed any modifications to the bike or is it stock?
Any info will help.
