ive had some experience with this. make sure to check and see if air bubbles air comming in when you have a full tank. When fuel levels are low fuel can bounce around and cause air bubbles to travel to the carb in some cases. last time i had problems with air bubbles i checked my remote needle valve and found fuel escaping from a crack. when you spin up your engine check to see if any fuel is being blown back from your remote needle valve. if so be sure to replace the assembly