Holley carburetors are a popular aftermarket upgrade for cars and trucks. The carburetor combines the car's fuel with air in the correct proportion for combustion. The lean-to-rich parameter adjusts the ratio of air to fuel that is combined inside the carburetor with each cycle of the engine. A leaner mixture produces less horsepower, but gives more miles to the gallon. A richer mixture provides more power, but burns more fuel and increases wear on engine components, potentially shortening their life span.
Park the car on a flat, level surface and lift the hood. Locate the carburetor between the air cleaner and the cylinders. Find the adjustment screw on the fuel metering block. It can be found between the main body of the carburetor and the bowl bolted to the side nearest the throttle level. The adjustment screws are on either side of the block.
Decide on the best way to access the screws. Depending on the layout of your engine, it may be easier to reach up from under the engine, or down through the hood. Start the engine and run it for at least five minutes to bring it up to temperature.
Allow the car to idle, then reach in and turn the screws. Each screw adjusts a pair of the four bowls. Turn each screw a quarter-turn in the same direction to increase or decrease the richness of the mixture.
Continue applying quarter-turns to both sides and listening to the engine. If the engine starts to misfire, the mixture is too lean. If the exhaust smoke becomes dark, then it is too rich. There is no correct position for fuel richness. Any point between the two extremes is acceptable, bearing in mind the impact it will have on performance and economy.
