How to Adjust a Carter BBD 2 Barrelby John Stevens J.D.
The BBD was a two-barrel carburetor manufactured by Carter during the 1960s and 1970s. The BBD was produced exclusively for Chrysler, Plymouth and Dodge vehicles. As with all carburetors, the BBD requires periodic adjustments. The idle mixture screws determine the amount of air and fuel distributed by the carburetor. Within the carburetor are two floats designed to ensure that the carburetor has a ready-supply of fuel to prevent hesitation. The dashpot is an emissions-reducing device that it important for smog control. These components should be adjusted as part of every tune-up.
Idle Mixture Screws
Turn the engine on and allow it to idle. The engine must idle at normal operating temperature while adjusting the idle mixture screws.
Turn the headlights on to the high-beam setting.
Place the transmission in neutral if the vehicle is equipped with an automatic transmission.
Insert the probe of a gas analyzer at least two feet into the tailpipe, then calibrate the analyzer according to the manufacturer's instructions.
Place a clamp anywhere along the length of the rubber vacuum tube connecting the distributor to the intake manifold.
Locate the two idle mixture screws on the bottom of the front of the carburetor.
Turn each mixture screw in a counterclockwise direction 1/16 of a turn with a screwdriver, then wait for at least 10 seconds and check the reading of the gas analyzer. The gas analyzer must indicate that the mixture is richer. Continue to turn the screws 1/16 of a turn and check the analyzer until the analyzer indicates a richer mixture.
Turn each mixture screw until the analyzer shows an air-fuel mixture reading of 14.2. Turn the screws in a counterclockwise direction to lower the analyzer reading or in a clockwise direction to raise the analyzer reading.
Remove the screws that secure the top of the carburetor to the body of the carburetor with a screwdriver. The floats are mounted against the underside of this top portion, called the "air horn."
Lift the air horn off of the carburetor and turn the air horn upside down to expose the floats.
Measure the distance between the bottom of the floats and the bottom of the air horn, meaning the portion of the air horn that touches the body of the carburetor when installed with a T-scale. The factory measurement depends on the model of the BBD as found on the tag mounted to the carburetor. The distance is 9/32 inches if the model number is 3240S, 3241S, 3244S or 3245S. The specification is 5/16 inches for model numbers 3684S, 3849S, 3850S, 4125S, 4126S, 4127S, 4128S, 4296S, 4297S, 4306S, 4307S, 4422S, 4423S, 4631S, 4614S and 4474S. The distance is 11/32 inches for model numbers 4725S, 4726S, 4727S, 4728S, 4894S, 4961S and 4962S. The distance for all other model number is 1/4 inches.
Bend the metal tabs at the base of the floats to adjust the distance.
Lower the air horn onto the carburetor and tighten the air horn's securing screws.
Dashpot (Manual Transmission Vehicles Only)
Adjust the idle mixture screws as described in Section 1.
Locate the dashpot at the front of the driver's side of the carburetor. The dashpot looks like a metal canister.
Pull the throttle linkage towards the dashpot just until the linkage touches the dashpot, then direct an assistant to read the vehicle's tachometer. The tachometer should read 2,000 rpm. The throttle linkage is the metal bracket on the driver's side of the carburetor that moves upward when the accelerator pedal is depressed.
Adjust the position of the dashpot until the tachometer reads 2,000 rpm. Loosen the lock nut on the tip of the dashpot with a wrench and turn the dashpot in a counterclockwise direction to lower the tachometer reading or in a clockwise direction to raise the tachometer reading, then tighten the locknut.
Items you will need
- Gas analyzer
- Carburetor T-scale
- Wrench (manual transmissions only)
- "Motor's Auto Repair Manual"; Ralph Ritchen; 1968
- "Motor's Auto Repair Manual, 39th Edition"; Motors; 1976