Thats a good one, first check that you haven't over filled the oil in the crank case, or if its been sitting a long time, that it isn't filled with water (Condensate). next, you might want to make sure the carb float(s) are not stuck overfilling the carbs. Thats really all it can be.
Does the engine stall after 3 seconds ?
Valves are to tight, I think they are shim over-bucket. Either way- you need to know how to adjust shim clearance and have a selection of shims and a bucket depression tool to change them. Best bet is to take it the the dealership and have a "major" service/tune - not "minor". They will go through the whole bike changing fluids and making adjustments including the valves. Plan on paying around $400 for the service.
Another possible problem - tight or leaking intake valves.
When the piston comes up on the compression stroke, it blows everything back through the intake valve into the air box.
Adjust the valves and perform a cylinder leak down test.