When a Quadrajet carburetor is running lean, it means that there's an imbalance in the air-to-fuel ratio, with too much air and too little fuel entering the engine's combustion chamber. Several factors can cause this issue:

• Incorrect Carburetor Adjustment: If the carburetor is not adjusted properly, it may not be delivering the right amount of fuel to match the airflow, leading to a lean condition. This would include the float adjustment and perhaps the choke adjustment.
• Vacuum Leaks: Leaks in the intake manifold or vacuum hoses can allow unmetered air to enter the engine, throwing off the air-fuel mixture and causing it to run lean. Use spray carb cleaner around any fitting, carburetor base and vacuum hoses.
• Dirty or Clogged Jets: Dirt, debris, or varnish buildup in the carburetor jets can restrict fuel flow, resulting in a lean mixture. This can happen when the carburetor sits for several months. The gas turns to varnish and coats the gas tank and inside the carburetor.
• Faulty Fuel Pump: A weak or failing fuel pump may not be delivering enough fuel to the carburetor, causing it to run lean. Test for 5 lbs prox.
• Malfunctioning Choke: A choke that doesn't close properly during cold starts can allow too much air into the carburetor, leading to a lean condition until the engine warms up.
• Restricted Fuel Supply: A kinked or clogged fuel line, or a partially blocked fuel filter, can restrict the flow of fuel to the carburetor, causing it to run lean.
• Incorrect Timing: Incorrect ignition timing can affect combustion efficiency, leading to a lean air-fuel mixture.

Addressing these potential causes through proper diagnosis and troubleshooting can help resolve the lean running condition in a Quadrajet carburetor, ensuring optimal engine performance and efficiency.