The vacumm that opens the end carbs is generated by airflow velocity thru the venturi's of the center carb. It is totally seperate and unrelated to manifold vacumm. The vac line to the end carbs hooks up to a nipple on a boss that is integral to the body of the carb. Like someone said, Base plate goes to choke pull off diaphragm, metering block to dist advance. If you think about it, if the end carbs worked off of manifold vac, they would work in reverse,as we all know manifold vac is high at idle and very low at full throttle. Enjoy your six pack!