NO ACCELERATION, SURGING SOMETIMES

New carb, vaporizor and lockoff. Tank was replaced.

There is a lower throttle plate under the carb with a vacum line on it that goes to the air inlet tube.
If I apply vacum and hold it the engine will now rev up.
The diaphram will not hold vacum.
I feel this is the faulty part but I cant identify it.
Its some sort of secondary throttle or govenor under the carb.
can someone please find me a part # and description. Thank you.