Vucuum advance does heaps. Dont compare webers /dellortoes to SU /hitachies. The SU's type carb works solely on the vucuum created on the engine side of the throttle plate in order to operate. Webers and Dellortoes have the "accelerator pump circuit to counteract the rush of air when the throttle plates is opened.

Typically the static timing is advanced when Weber /dellortoes carbs are fitted..

Also if pepper has not blocked off the vacuum port on the front hitachi then that carb will see a vucumm leak. If the leak is present then the air rushing over the bridge, where the jet sits will be reduced. This condition will induce a lean mixture condition. The twin carbs will be out of balance also.