Fuel System:
* Vacuum Leaks: A significant vacuum leak can cause erratic idle and affect the choke (which is often vacuum-operated). Leaks can be anywhere in the intake manifold, vacuum lines, or even the power brake booster. This is a highly likely culprit given the symptoms.
* Clogged Fuel Filter: Restricted fuel flow can cause sputtering and poor idling, especially under load when the engine demands more fuel.
* Fuel Pump Issues: A weak fuel pump might supply enough fuel at higher RPMs (under acceleration) but struggle at idle. This could also lead to sputtering.
* Carburetor Problems: Given the age, carburetor issues are very likely. This includes:
* Dirty or faulty carburetor: Dirt, debris, or a malfunctioning float level can affect fuel delivery. A rebuild or replacement might be needed.
* Stuck or malfunctioning choke: Although it's activating, it might be sticking open or closed at the wrong times, causing the idle issues.
* Clogged jets or passages: Similar to a clogged fuel filter, this restricts fuel flow.
* EGR Valve: A faulty EGR (Exhaust Gas Recirculation) valve can cause rough idling and poor performance.
Ignition System:
* Distributor Issues: Worn distributor cap, rotor, or points (if equipped with points, not electronic ignition) can lead to misfires and poor running.
* Worn Spark Plugs or Wires: These components are crucial for proper combustion. Worn parts can cause misfires, particularly under load.
* Ignition Coil: A failing ignition coil can struggle to provide enough voltage at idle, but might be sufficient under higher RPMs.
* Timing Issues: Incorrect ignition timing can lead to poor idling and sputtering.
Other Possibilities:
* Oxygen Sensor (if equipped): While less likely on a 1980 model, a malfunctioning oxygen sensor could affect fuel mixture and trigger the check engine light.
* Temperature Sensor: An inaccurate temperature sensor can send incorrect data to the computer (if equipped with a rudimentary computer system) leading to incorrect fuel mixture.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum lines and the intake manifold for cracks or loose connections. Use a vacuum gauge to check manifold vacuum at idle.
2. Inspect the Carburetor: Check the float level, clean the jets, and make sure the choke is operating correctly. A rebuild kit might be a good investment.
3. Check Spark Plugs and Wires: Inspect for wear, fouling, or damage. Replace if necessary.
4. Check the Distributor: Inspect the cap and rotor for wear or cracks. Replace if needed. If points are used, check their condition and gap.
5. Test the Fuel Pump: Check the fuel pressure.
6. Replace Fuel Filter: A cheap and relatively easy first step.
Important Note: The check engine light in a 1980 model is likely a very basic warning, not providing specific diagnostic codes like later systems. You'll need to diagnose the issue through systematic testing of the components listed above. A mechanic specializing in older vehicles will have the knowledge and tools to efficiently diagnose this. Trying to jump to replacing parts randomly can be expensive and unproductive.
