How to Tune a Carburetor Through the Use of the Manifold Vacuum of a V-8 Internal Combustion Engine

Properly used, a vacuum gauge can improve mileage and power.

Eight-cylinder internal-combustion engines rely on vacuum to draw the air/fuel mixture from the carburetor to each cylinder via an intake manifold. The vacuum is created when a piston moves downward in its cylinder. Measuring the vacuum with a vacuum gauge can provide a wealth of knowledge about the health of the engine, including the carburetor, without requiring disassembly. The gauge is fairly inexpensive and straightforward to use.

Things You'll Need

• Vacuum gauge
• Screwdriver
• Engine service manual

Instructions

1. Locate a vacuum port on the intake manifold. Intake manifolds typically feature a fitting that screws into the top of the manifold. Several metal nipples are located at the top of the fitting. These nipples are vacuum ports.

2. Connect a vacuum gauge to an unused vacuum port on the intake manifold. Note that some vacuum ports have a rubber hose attached to them, while others feature only rubber caps. Pull the cap off of one vacuum port, and plug the hose that extends from the bottom of the vacuum gauge onto that port.

3. Start the engine and observe the gauge needle while an assistant gradually increases and decreases the speed of the engine with the accelerator pedal. If the needle gradually drifts back and forth rather than holding steady, the carburetor most likely requires adjustment.

4. Turn the carburetor's air/fuel mixture screw(s) with a screwdriver while the engine idles until the vacuum gauge needle steadies. If the needle does not steady when the screws are turned in one direction, turn the screws in the opposite direction. Note that some carburetors are equipped with one air/fuel mixture screw and others are equipped with two. The mixture screws are usually located at the base of the front of the carburetor. Consult the engine's service manual to locate the screws if necessary.

5. Turn the idle-speed screw on the driver's side of the carburetor in a counterclockwise direction to lower the idle speed to its factory setting, as determined by the service manual.

6. Turn the engine off and unplug the vacuum gauge from the vacuum port, then press the rubber cap onto the port.

References

• "Motor's Auto Repair Manual"; Ralph Ritchen; 1968