The Carter BBD carburetor features an idle circuit that is responsible for controlling the engine's idle speed and air-fuel mixture. This circuit consists of various components, including idle jets, idle mixture screws, and idle air bypass valves. Proper adjustment and maintenance of the Carter BBD idle circuit are crucial for optimal engine performance.

ADJUSTMENTS

FAST IDLE LINKAGE

With choke valve tightly closed and lug on fast idle cam contacting lip on choke shaft lever align center of fast idle screw with index mark on cam. To adjust, bend lip on choke shaft lever.

FAST IDLE THROTTLE VALVE

1. Be sure choke valve is closed

2. Adjust fast idle adjusting screw against index mark of cam (revolve cam if necessary) until the clearance between throttle valve (side opposite port) and bore is as listed on carburetor specification page.

IDLE MIXTURE SCREW ADJUSTMENT

• Bring the engine to operating temperature.
• Adjust the idle to manufactures specifications.
• NOTE: Having the idle up too high will bypass the idle circuit and adjusting the idle mixture will have not affect.
• Hook up a vacuum gauge to a port that has steady vacuum.
• Turn both screws out 1/2 turn - Turn the idle mixture screw out 1 1/2 turns while on the bench.
• Turn each screw in, alternating between them, 1/4 turn at a time until you get a steady vacuum reading.

Another option:

• Bring the engine to operation temperature.
• Adjust the idle to manufactures specifications.
• Turn both idle mixture screws out 1/2 turn.