My initial thoughts is that it is fuel starved. If you have an on/off gas valve, remove the carburetor end and then turn in the fuel valve to verify fuel flow. If it flows properly put it back on. Try starting again. If same problem, try playing with choke and see if it's a fuel mixture issue. Was the gas in the tank new gas or same gas as was previously ridden with? Check to see if any of the carburetor mounting hardware needs tightening. If still same symptoms, pull spark plug and see if it's wet with fuel or not. If wet dry off plug and retry. If dry try using the choke a bit longer. Let us know your results.