A Carter YF or YFA carburetor running rich at idle could be caused by several issues. Here are the most common reasons:

1. Float Level Too High

• If the float is set too high, it allows excess fuel into the carburetor bowl, leading to a rich mixture.
• Solution: Check and adjust the float level according to the manufacturer's specifications.

2. Stuck or Worn Needle Valve

• The needle valve controls fuel flow into the bowl. If it's stuck open or worn, it can cause excessive fuel to enter.
• Solution: Inspect, clean, or replace the needle valve if necessary.

3. Choke Malfunction

• If the choke is stuck partially closed, it will cause a rich condition.
• Solution: Ensure the choke plate opens fully once the engine warms up. Check for proper adjustment or issues with the choke mechanism.

4. Idle Mixture Screw Adjustment

• If the idle mixture screw is set too rich, it will cause excessive fuel at idle.
• Solution: Adjust the screw by turning it in (lean) or out (rich) until you achieve the highest vacuum reading or smoothest idle.

5. Dirty or Clogged Air Filter

• A restricted air filter limits airflow, enriching the mixture.
• Solution: Inspect and replace the air filter if it's dirty.

6. Vacuum Leaks (Opposite Effect)

• Normally, vacuum leaks cause a lean condition, but if the leak is in an area that disrupts the carb's ability to meter fuel correctly, it could result in uneven idle behavior that feels rich.
• Solution: Inspect vacuum lines, gaskets, and carb base for leaks.

7. Worn Throttle Shaft