HOW IT WORKS
The Carter BBD usually includes the choke thermostat and usually a choke pull-off is a 2 barrel carburetor which generally uses a divorced choke type of thermostat. This means the choke thermostat is mounted in the intake manifold and is connected to the carburetor choke valve by a choke rod.
When the engine is cold, the choke valve should be closed. You may have to open the throttle a bit so that it can close. The fast idle cam may be holding it.
As the engine gets hot, the thermostat will gradually open the choke valve.
A lot of the Carter BBD carburetors will have a choke pull-off attached to the carburetor. This is used to open the choke valve slightly when the engine is cold and you want to quickly accelerate. At idle the choke pull-off piston is pulled back by vacuum. As you stomp on the gas pedal, the vacuum drops allowing the piston to move out and when far enough will open the choke valve slightly (when cold), letting in some air. This is so that the carburetor doesn't flood out when cold.
Watch the choke valve. When cold it should be closed. When hot, it should be fully opened. Again, you may have to move the throttle slightly in case the fast idle cam is holding the choke.
FAST IDLE CAM
This is the fast idle cam on the early BBD carburetor. When the engine is cold and the choke is closed, the fast idle cam moves so that the high step is on the idle screw. This increases the RPM so that the engine doesn't choke out. As the choke warms up, the lever moves to smaller and smaller points. To adjust - Adjust the warm idle 1st, then put the fast idle cam on the high step and adjust the idle screw to the RPM recommended for your engine.
TYPICAL DIVORCED STYLE CHOKE
ADJUSTMENT
Loosen mounting post lock nut and turn mounting post with screwdriver until index mark on disc is positioned as listed in your spec chart. Hold in this position with screwdriver and lock nut.
After adjustment is completed and coil housin, rod, and carburetor are installed on engine, lift cover disc and open and close choke valve manually to see if connector rod clears sides of hole in housing cover without binding.
If rod does not clear housing cover without binding, replace with a new unit since connecting rod cannot be bent without affecting calibration.
ADJUSTMENTS
INITIAL CHOKE VALVE CLEARANCE ADJUSTMENT
1978 American Motors
- Loosen choke cover and turn 1/4 RICH - Due to 60 years of were you may have top play around with this.
- Tighten one retaining screw to hold cover in place
- Open throttle valve slightly
- Place fast idle screw on HIGH step of fast idle cam
- Using an outside vacuum source (at least 19in Hg) apply vacuum to diaphragm assembly to pull diaphragm in against stop
- Measure clearance between upper edge of choke valve and air horn wall
- If clearance is not within specifications, adjust by bending diaphragm connector link at "U" shaped portion
CHOKE VACUUM KICK ADJUSTMENT
1978 Chrysler Corp
- With engine OFF, open throttle and close choke. Now close throttle to trap fast idle cam at closed choke position
- Apply outside vacuum source (at least 15 in. Hg) to carburetor choke diaphragm assembly
- Apply enough closing force on choke lever to fully compress spring in diaphragm without distorting linkage
- Measure clearance between upper edge of choke valve and air horn wall on throttle lever side
- If clearance is not within specifications, adjust by bending diaphragm link at "U" shaped area (open or close "U")
- Check linkage after adjustment for binding or parts interference
- Remove vacuum source, reconnect vacuum hose
CHOKE UNLOADER ADJUSTMENT (WIDE OPEN KICK)
1978 All models
- Hold throttle wide open. Insert proper size drill bit or pin gauge between upper edge of choke valve and air horn wall
- Apply light closing pressure with finger on choke valve. Slight drag should be felt on gauge as it is withdrawn. If not, adjustment is needed
- Adjust by bending unloader tang on throttle lever. Use care that tang will not interfere with other parts after adjustment
Images of a later BBD choke system:
