Well, you have done it all right this far, my sudgestion was goin to be a fuel pump OR your engine is not geting enough air. Try taking off your air bow and see, then make sure the carbs both open and shut as per usual, then runn it again. Do you have reeds? if you do maybe its one of those is broke or stuck. Try those and then we will go from there.

Obviously it isnt geting air or fuel or both, the question is why. let me know.
dano