AFB - Adjusting Idle Mixture Isn't Changing the RPM

If adjusting the idle mixture screws, or on some older AFB models the center screw on an AFB carburetor isn't affecting the engine RPM, there are a few possible causes worth checking:

Possible Issues:

1. Vacuum Leak

   • A vacuum leak downstream of the carburetor (like at the intake manifold or a vacuum hose) can cause unregulated air to bypass the idle circuit, making mixture screw adjustments ineffective.

   • Check for: Hissing sounds, use carb cleaner or propane around gasket areas to detect RPM changes.

2. Idle Speed Set Too High

   • If the throttle plates are open too far (because the idle speed screw is turned in too much), the engine may be running on the main circuit instead of the idle circuit.

   • Solution: Back off the idle speed screw and re-adjust the idle mixture screws.

   • Another possibility is if you removed the throttle plates during cleaning you may not have gotten them back in correctly. Usually the trademark can be seen on the plate when turning the carburetor upside down. Also look to see if there is open areas with idle closed.

3. Clogged Idle Circuit

   • The idle passages or ports in the carburetor could be clogged, preventing fuel from flowing even when the mixture screws are adjusted.

   • Solution: Remove the screws and spray carb cleaner through the passages. You may need to disassemble and clean the carb thoroughly.

   • If the vehicle sits for a long period of time then the gas may have turned to varnish coating inside the carburetor, fuel lines & gas tank.

4. Faulty or Misadjusted Float Level

   • An incorrect float level can affect fuel delivery to the idle circuit.

   • Check: Float height specs and fuel level in the bowls.

5. Worn Throttle Shaft or Base Gasket Leaks

   • Air leaks around the throttle shaft bushings or carb base can also bypass the idle circuit.

   • Check: Spray carb cleaner around the throttle shaft and base—RPM change indicates a leak.

6. Incorrect Timing

   • Improper ignition timing can mask the effect of idle mixture adjustments.

   • Check: Ensure base timing is correctly set before adjusting idle mixture.

Basic Troubleshooting Steps:

1. Set base timing correctly.

2. Set idle speed screw to just barely open the throttle plates.

3. Adjust mixture screws for highest RPM (or vacuum, if using a gauge).

4. Re-check idle speed and timing as needed.