Carburetion Issues (Most Likely):
* Fast Idle Cam: This mechanism is designed to raise the idle speed when the engine is cold to aid in warming up. If the fast idle cam isn't returning to its normal position after the engine warms up, the idle will remain high. This often involves a sticking linkage or a faulty thermostat.
* Vacuum Leaks: Leaks in the vacuum system can cause erratic idling and high idle speeds. Check all vacuum lines connected to the carburetor and intake manifold for cracks, looseness, or disconnections. Pay close attention to the PCV valve (positive crankcase ventilation) as well.
* Choke Problems: If the choke is sticking or not returning fully to the open position, the engine will receive too much fuel, resulting in a high idle. Manually operating the choke can help determine if this is the culprit.
* Dirty or Malfunctioning Carburetor: Over time, carburetors can become clogged with dirt and debris. This can affect fuel delivery, leading to a high idle. A thorough cleaning or rebuilding might be necessary.
* Incorrect Idle Mixture Adjustment: The idle mixture screws on the carburetor control the air/fuel ratio at idle. If they're incorrectly adjusted, the idle speed can be affected.
Ignition System Issues:
* Worn Distributor Points (if applicable): In cars with points-type ignition, worn points can cause erratic timing and a high idle.
* Vacuum Advance Problems: The vacuum advance mechanism in the distributor adjusts the ignition timing based on engine vacuum. A malfunctioning vacuum advance can also lead to a high idle.
* Ignition Timing: Incorrect ignition timing can significantly affect idle speed.
Other Possibilities:
* Throttle Linkage: A sticky or improperly adjusted throttle linkage can prevent the throttle plate from closing completely, resulting in a high idle.
* Intake Manifold Gasket Leak: A leak in the intake manifold gasket can cause a lean air-fuel mixture and affect idle.
* Temperature Sensor (Thermo-Time Switch): A faulty temperature sensor can cause problems with the idle system if it's malfunctioning and not relaying appropriate information about engine temperature.
Troubleshooting Steps:
1. Check the obvious: Listen carefully to the engine. Are there any unusual noises? Visually inspect the carburetor, vacuum lines, and throttle linkage for obvious problems.
2. Manual Choke Operation: Try manually operating the choke to see if it affects the idle.
3. Check Vacuum Lines: Carefully inspect all vacuum lines for leaks, cracks, or disconnections.
4. Listen for Vacuum Leaks: While the engine is idling, spray carburetor cleaner around suspected vacuum leak areas. A change in idle speed indicates a leak.
5. Consult a Repair Manual: A repair manual specific to your 1968 car model will provide detailed information on the carburetor, ignition system, and other components, including adjustments and troubleshooting steps.
If you're not comfortable working on the car yourself, it's best to take it to a mechanic specializing in older vehicles. They'll have the experience and tools to diagnose and fix the problem efficiently. Describing the symptoms accurately (high idle only when starting in Park, engine temperature, etc.) will help them pinpoint the issue quickly.
