How to adjust the carburetor on a 1968 Ford Mustang GT Fastback (Bullitt)?

If your 1968 Ford Mustang GT Fastback (Bullitt) is experiencing rough idling, poor acceleration, or excessive fuel consumption, it might be time to adjust the carburetor. The carburetor, which controls the mixture of air and fuel entering the engine, can often lose its optimal settings due to wear and age. Ensuring that your carburetor is finely tuned will enhance performance and fuel efficiency. Here is how to solve this issue:

Step 1: Preparation

Before you begin, make sure your Mustang is parked on a flat surface and the engine is turned off. Open the hood and locate the carburetor, which is situated on top of the engine, usually capped with an air filter. Remove the air filter to gain clear access to the carburetor.

Step 2: Identify Adjustment Screws

The carburetor will have two main adjustment screws: the idle mixture screws and the idle speed screw. The idle mixture screws typically have springs around their bases and are found on the side of the carburetor. The idle speed screw is often located near the throttle lever of the carburetor.

Step 3: Adjust Idle Mixture Screws

Using a flat-head screwdriver, gently turn the idle mixture screws clockwise until they lightly seat. Be careful not to overtighten, as this could damage the screws. Once both screws are seated, turn them counterclockwise one and a half turns as a starting point. These screws control the air-fuel mixture at idle, so fine-tuning them is crucial for smooth engine performance.

Step 4: Adjust Idle Speed Screw

Locate the idle speed screw, generally found near the throttle lever. Turn this screw clockwise to increase the idle speed and counterclockwise to decrease it. The goal is to set a stable idle speed—typically around 600-700 RPM for a 1968 Mustang GT Fastback. You’ll need a tachometer to measure the engine speed accurately.

Step 5: Warm Up the Engine

Start the engine and let it reach its normal operating temperature. This usually takes around 10-15 minutes. Pay attention to the engine’s response; if it’s sputtering or idling roughly, you may need to readjust the screws slightly.

Step 6: Fine-Tuning

With the engine warm, go back to the idle mixture screws. Turn each screw slowly clockwise and counterclockwise to find the point where the engine runs most smoothly. Adjust each screw gradually; small increments can make significant differences. After optimizing the mixture, recheck the idle speed and adjust the idle speed screw if necessary.

Step 7: Reinstall Air Filter

Once you’re satisfied with the adjustments, turn off the engine and replace the air filter. Ensure it is secured properly to prevent any air leaks that might disrupt the carburetor’s performance.

Step 8: Test Drive

Take your Mustang for a test drive to ensure that the adjustments have improved performance. Pay attention to the idle at stoplights, acceleration, and overall power. If needed, you can perform minor tweaks once you return.

By following these detailed steps, you should be able to improve the performance and fuel efficiency of your 1968 Ford Mustang GT Fastback (Bullitt). Regularly checking and adjusting the carburetor can help keep your classic car running smoothly.