Here's why:
* Cold Start Enrichment: The automatic choke enriches the fuel mixture when the engine is cold. This is crucial for starting and stable idling until the engine warms up.
* Warm-Up Transition: As the engine warms, the thermostat in the choke gradually opens, reducing the amount of fuel going into the engine. If this transition isn't working correctly, the mixture becomes too lean when warm, causing stalling.
Other possibilities, though less likely given your description:
* Vacuum Leaks: A significant vacuum leak can cause idling problems, especially as the engine warms and demands more air. Check all vacuum lines for cracks, loose connections, or holes. Pay close attention to the lines connected to the distributor and the carburetor itself.
* Fuel Delivery: While less probable if it idles fine cold, a partially clogged fuel filter or issues within the fuel pump could contribute.
* Ignition System: Weak ignition points, a failing condenser, or a worn distributor could also cause idling problems, but this is less likely if it idles well cold.
* Carburetor Adjustments: The carburetor itself might need adjustment; however, since it idles fine cold, this is less likely than the choke.
Troubleshooting Steps:
1. Inspect the Automatic Choke Thermostat: Visually inspect the choke mechanism for any obvious damage, binding, or corrosion. Look for smooth operation of the choke plate as it warms. A simple test is to carefully feel the thermostat housing, it should warm up progressively as the engine runs.
2. Check for Vacuum Leaks: Carefully inspect all vacuum lines, paying attention to those connected directly to the carburetor. Use a vacuum gauge (if you have one) to check for proper vacuum at idle.
3. Manual Choke Test (if applicable): If your car has a manual choke override, try using it to see if enriching the mixture when warm helps the idle. This is NOT a permanent solution, but a quick test.
4. Clean the Carburetor: A thorough cleaning might help, addressing potential minor blockages or issues in the jets, but this is a more involved task.
5. Replace the Automatic Choke Thermostat: If the visual inspection shows damage or the choke isn't functioning correctly, replacing the thermostat is usually the most effective solution. This part is relatively inexpensive and readily available for your model.
Remember, working on carburetors can be tricky. If you're not comfortable with this type of repair, take it to a mechanic specializing in older VWs. They will have the experience and tools to diagnose and fix the problem efficiently.
