Primary Vacuum Break

NOTE: Choke coil lever adjustment must be correct before performing this adjustment. This adjustment is performed using the choke angle gauge, see procedure at beginning of Adjustment.

Using an outside vacuum source of at least 15 in. Hg, seat primary vacuum break diaphragm. Plug purge bleed hole (if equipped) with masking tape. Hole is found in end of diaphragm.
On delay models with air bleed, remove rubber cover over filter element and plug bleed hole in vacuum tube with masking tape. On models with air bleed in end cover, plug cover with masking tape. Remove tape after adjustment.
Air valve rod must not keep vacuum unit from fully retracting. If necessary, bend air valve rod at air valve lever to provide clearance for proper adjustment. Adjust final air rod clearance after setting primary vacuum break adjustment.
Lightly close choke by pushing up on choke coil lever or vacuum break lever tang. Hold choke closed with a rubber band. Make sure bucking spring on diaphragm plunger (if equipped) is compressed and seated. Bubble on angle gauge should be centered with specified degree mark opposite pointer. See Fig 1.

Fig 1: Primary Vacuum Break Adjustment
Courtesy of GENERAL MOTORS COMPANY

Lightly hold choke in closed position.
To adjust, turn vacuum break adjustment screw in until bubble of choke valve angle gauge is centered. Remove gauge.
NOTE: Some models will have tamper-proof plugs over the adjustment screw. To gain access to adjustment screw, remove vacuum break bracket from carburetor. Carefully grind off plugs over adjustment screw and replace vacuum break diaphragm.