Well, start at the 10,000 ft view... an engine needs fuel, air, and spark to run. So, I'd try flooding it to make sure you're getting fuel, then I'd pull the plugs to make sure you're getting spark. Or probably better to do the plugs first, I dunno - if you flood it then pull the plugs, wet plugs would indicate you're getting fuel, but if you pull the plugs first, you might be able to just smell gas from the plug hole. If you're getting spark and fuel... something must be choking the airbox. Figure it out from there. And check the dumb stuff first...like that your petcock isn't off or on reserve.