pull your carbies off take bowl off and clean out gunk build up in there you will have crap in there blocking main jet when the flow increases when you back off they drop back to bottom of bowl..had same trouble with my 60yammie took all three carbs off cleaned them put water separator filter on and havnt prob since...unleaded fuel leaves deposits when it dries out...just use some carby cleaner and spray thru the ports in your carbs while apart..rick