Most likely a gummed up carb. I just ran through this with my wife's scooter -- had sat for several months, would start then die in a few seconds. I dumped half a can of SeaFoam into the gas tank and started up the engine several dozen times (enough to get gas from the tank down to the carb. Then I *carefully* took the top off the carb, lifted out the diaphragm, and dripped a couple of capfulls of SeaFoam down into the needle jet hole (to get the seafoam down into the carb bowl). The bike will start and run without the diaphragm, but (obviously) won't respond the throttle. After a few more start/die cycles, it started to hold the idle. Put the carb back together and it seems to run fine now.
If you're lucky (as I was) this "external" flush will do the job. If not, you'll have to pull the carb to get at the jets to clean them out.
Edit: Make sure you have a battery charger connected. Those repeated starts will kill your battery charge in a hurry.