How to Troubleshoot a Holley One-Barrel Carburetorby Kelvin Hayes
The Holley one-barrel carburetor is the simplest of the Holley designs, with one intake bore, one throttle plate and one fuel delivery system. Because of this simplicity, troubleshooting will take less time, but the one-barrel will experience the same problems as a two-, three- or four-barrel carburetor. Before tearing down the carburetor, first explore and repair all other possible causes of your vehicle's engine problems.
Remove the sight port plug screw from the side of the fuel bowl and check the fuel level.
Wiggle the car slightly to move the fuel within the fuel bowl. You're looking for a slight trickle to come out of the sight port.
Adjust the float level adjustment screw clockwise to lower the float, reducing the amount of the fuel in the bowl. (ideal for flooding conditions which often cause an engine to fail to start).
Locate the slow idle mixture screws on the sides of the metering block (two screws total). The metering block is the small section of the carburetor that attaches the fuel bowl to the throttle bore.
Screw them until they are tightened. Do not over-tighten. Back the screws out exactly 1.5 turns to provide a common starting point for your adjustment.
Start the engine and adjust the screws equally counterclockwise until the engine idle smooths out, slows down and reaches a sustainable RPM range as specified in your engine's service manual.
Remove the air filter cap wing nut, filter cap, filter and base to expose the choke tower and carburetor bore.
Start the car and operate the accelerator cable while watching the carburetor bore. Ensure that the fuel delivery nozzle provides fuel to the top of the throttle plate throughout the acceleration process without over-delivering. Excess fuel on the top of the throttle plate before and after acceleration will indicate too much fuel is entering the bore, causing sluggish acceleration.
Unscrew the fuel pump nozzle screw with a screwdriver and pull the nozzle out of the bore using needle-nose pliers.
Insert a nozzle three sizes smaller than the one removed from the bore. Insert the screw and tighten slightly. Start the car again and test the acceleration for a smooth, crisp feel and sound.