the bowls of the carbs need to fill up with gas. on the pregens, this is a relatively difficult thing to do once the carb bowls are emptied. the petcock is vacuum operated and won't flow fuel unless the engine is started. the engine won't start w/o fuel in the bowls.
easiest way is to connect a piece of hose to the vacuum port on the backside of the petcock and gently suck on the hose to create a vacuum. this will allow the fuel to flow down into the carbs. a vacuum gun will work too, if you have one.
some background info on the fuel petcock...
http://faq.ninja250.org/wiki/Rebuild...tcock/fuel_tap