I'm thinking along the lines of Hotrod lincoln. When you open both butterflys your vaccum drops off to quickly before the engine rpm's come up, the venturi effect is lost. Thats what accelerator pumps are supposed to compensate for. Be sure yours throw a good spray and are set for the longest stroke possible if yours are adjustable like old Strombergs are. Raising the fuel level in the bowl my help a little too. Pop ing back thru the carburetor is a sure sign of a lean mixture.
