* Idle Air Control (IAC) Valve (if equipped): While less common on carburetors compared to fuel-injected systems, some later-model carburetors might incorporate a rudimentary IAC or similar device to control idle speed. A malfunctioning IAC could fail to compensate for the change in airflow when the engine is idling, causing the engine to stall when load is reduced. This is less likely than the other options given the age and simplicity of the system, but still possible.
* Vacuum Leaks: A significant vacuum leak will affect the engine's ability to maintain a stable idle, especially when the engine load decreases (like coming to a stop). Look for cracks or loose connections in the vacuum hoses running to the carburetor, distributor (if vacuum advance is used), and other vacuum-operated components. Small leaks may not be noticeable at higher RPMs, but become critical at idle.
* Fuel Pump/Fuel Delivery: While it starts right back up, the fuel pump might be weak. When the engine is idling, it demands less fuel. When stopping, the engine load drops dramatically, and a weak fuel pump might not be able to maintain sufficient pressure to keep the engine running. The restart immediately suggests the fuel delivery isn't completely blocked, but is insufficient at low engine speeds.
* Carburetor Issues: The carburetor itself could be the primary source of the problem. Several possibilities exist:
* Clogged idle jets or passages: These tiny passages deliver fuel at idle speeds. If they're clogged, the engine won't get enough fuel to stay running when the load is reduced.
* Float level too low: The fuel level in the carburetor's float bowl might be too low, leading to insufficient fuel supply at idle.
* Diaphragm issues: Issues within the carburetor's various diaphragms (accelerator pump, etc.) could cause erratic fuel delivery.
* Choke malfunction (if applicable): If the choke isn't fully disengaged when warm, it can restrict airflow and lead to stalling at idle.
* Distributor (if applicable): Although less likely given the symptoms, a failing distributor could cause issues with ignition timing, which might exacerbate the problem. A weak ignition system will struggle more at idle.
Troubleshooting Steps:
1. Check all vacuum hoses: Carefully inspect every vacuum hose for cracks, leaks, or loose connections. Replace any suspect hoses.
2. Listen for unusual sounds: Listen carefully for any unusual sounds emanating from the fuel pump when the engine is running. A weak or failing pump will often make a whining noise.
3. Examine the carburetor: Visually inspect the carburetor for any obvious signs of damage, leaks, or dirt. If you're mechanically inclined, consider a basic carburetor cleaning, focusing on the idle jets and passages. (This requires carburetor-specific knowledge and tools.)
4. Check the fuel filter: A clogged fuel filter can restrict fuel flow, especially at low demand. Replace it if necessary.
5. Professional Help: If you're not comfortable working on the carburetor or fuel system, take the truck to a mechanic specializing in older vehicles or carburetors. Diagnosing this could require specialized tools and experience.
The immediate restart strongly suggests it's not a major ignition or starting system issue. Focus your efforts on fuel delivery and idle control. Remember safety precautions when working with fuel systems.
