Carburetion Issues:
* Idle Mixture Screws: These screws control the air/fuel ratio at idle. They're likely too far out (too rich). They need careful adjustment – often a tiny turn makes a big difference. You'll need a vacuum gauge and/or a CO meter to properly tune them. This is the most likely culprit.
* Vacuum Leaks: Leaks in the vacuum system can cause erratic idle and a rich mixture. Check all vacuum lines for cracks, looseness, or disconnections. Pay particular attention to lines connected to the carburetor.
* Choke: If the choke isn't opening fully when the engine is warm, it can cause a rich condition at idle. Make sure the choke is operating correctly and fully opening when the engine is warmed up.
* EGR Valve (Exhaust Gas Recirculation): A malfunctioning EGR valve can affect the air/fuel mixture, particularly at idle. Check for proper operation and ensure it's not sticking or leaking.
* Fuel Pressure: Too much fuel pressure will lead to a rich mixture. Check the fuel pressure regulator (if equipped).
* Clogged Fuel Filter: A clogged fuel filter restricts fuel flow, potentially causing erratic behavior and a rich mixture, although this is less likely to cause specifically high idle CO and HC.
* Carburetor Needs Rebuild/Replacement: After checking all the above, the carburetor itself might be worn out or have internal issues affecting metering. A rebuild or replacement might be necessary.
Ignition Issues (less likely, but can contribute):
* Ignition Timing: Incorrect ignition timing can affect combustion efficiency, potentially leading to higher emissions. Have the timing checked and adjusted to factory specifications.
* Spark Plugs and Wires: Worn spark plugs or faulty ignition wires can cause incomplete combustion, resulting in higher HC emissions. Inspect and replace if necessary.
* Distributor Cap and Rotor: Check for cracks or wear on the distributor cap and rotor. These can cause arcing and poor ignition.
Troubleshooting Steps:
1. Start with the simplest: Check for obvious vacuum leaks first. Spray carb cleaner around vacuum connections while the engine is idling; a change in idle speed indicates a leak.
2. Adjust the idle mixture screws (carefully!): This requires patience and the right tools. Small adjustments are key. Consult a repair manual for your specific carburetor.
3. Check the choke operation: Make sure it opens completely when warm.
4. Inspect the EGR valve: Check its operation and for leaks.
5. Check fuel pressure (if applicable).
6. Replace spark plugs and wires (if necessary).
7. Inspect the distributor cap and rotor.
8. Consider a professional: If you're not comfortable working on the carburetor, take it to a qualified mechanic.
Important Note: Before adjusting anything, note the current settings of your idle mixture screws so you have a baseline to return to if needed. Improper adjustments can worsen the problem. Consult a repair manual specific to your 1976 Ford 351M for detailed instructions and specifications.
