ours did the exact same thing, it was using a ton of gas and blowing gas back into the airbox through the vent line. It ended up being a bad diaphragm in the fuel pump. Fuel was just running out of the vent line. I originally thought it was the floats, but i set them to spec and it still did it just as bad. Check your fuel pumps. If you pull the fuel lines off of the carbs and gas runs out of them, there is a bad diaphragm.