With engine running at normal operating temperature (160*F) and choke button all the way in, turn idle mixture adjusting screw counter-clockwise for richer mixture and clockwise for leaner mixture, and maintain engine idle speed of 450 to 500 RPM

If a satisfactory engine idle cannot be obtained by turning the idle mixture adjusting screw, check all other related engine adjustments before condemning carburetor.

Carter BB-1 Idle Mixture Adjustment

Adjusting the idle mixture on a Carter BB-1 carburetor is essential for ensuring smooth engine operation at low RPMs. Follow the steps below for proper adjustment:

Safety First: Always ensure the vehicle is in a well-ventilated area, and the engine is turned off and cool before starting adjustments.

1. Initial Preparation:
   • Make sure the air cleaner is properly installed and is not obstructed.
   • Ensure all vacuum lines are correctly connected and free of leaks.
   • Bring the engine up to its normal operating temperature by letting it run for several minutes.
2. Set Idle RPMAdjust the idle screw to get 300 RPM, or close to that.
3. Locate the Idle Mixture Screw:
   • On the Carter BB-1, the idle mixture screw is typically located on the side of the carburetor. It's a smaller screw, often near the base.
4. Baseline Adjustment:
   • Gently turn the idle mixture screw clockwise until it softly seats. Do not over-tighten as this can damage the needle.
   • Next, turn the screw counter-clockwise approximately 1 to 1 1/2 turns as a general starting point.
5. Adjustment:
   • Start the engine and allow it to idle.
   • Slowly turn the idle mixture screw in (clockwise) until the engine RPM starts to drop or the engine begins to stumble.
   • Now, slowly turn the mixture screw out (counter-clockwise) until you achieve the smoothest and highest idle RPM.
   • Once you achieve this peak RPM, turn the screw back in (clockwise) just a little until the RPM slightly reduces. This setting is typically the optimal idle mixture.
   • Another way is to connect a vacuum meter to the intake and adjust the idle mixture screw to get the smoothest RPM.
6. Recheck Idle Speed:
   • After adjusting the idle mixture, you may need to adjust the idle speed using the throttle stop screw to ensure it's at the manufacturer's recommended RPM.
7. Fine-tuning:
   • If the engine hesitates or stalls when accelerating from idle, the mixture might be too lean. Adjust the screw slightly out (counter-clockwise).
   • If the engine emits a rich exhaust smell or black smoke at idle, the mixture might be too rich. Adjust the screw slightly in (clockwise).
8. Final Check:
   • Make sure the engine transitions smoothly from idle to higher RPMs. This will ensure the idle mixture is set correctly and working in tandem with other adjustments.
   • Once you're satisfied, ensure all tools are removed, and the carburetor area is clean.

Remember, the idle mixture adjustment affects how the engine runs at idle and just off idle. Regular maintenance checks and adjustments will help keep your engine running optimally. If you're uncertain or lack experience, it might be beneficial to seek assistance from an expert or a relevant workshop manual.