One of the most crucial, yet overlooked adjustments in a Holley four-barrel carburetor is the float.
First, it’s important to know how the fuel float system operates. The floats, located in the fuel bowls of Holley four-barrel carburetors, control the fuel delivery system and ensure that the carburetor has a sufficient fuel supply. Some of the most common symptoms of improperly adjusted or sticky floats are hesitation on acceleration, a rough idle, flooding and misfires. Obviously, making precise fuel float and in turn, fuel level adjustments is difficult on a misfiring or rough idling engine. So the procedure to adjust this has to begin without the engine running.
Adjusting The Fuel Level:
Step 1: Fill the carburetor’s fuel bowls with fuel.
For a mechanical fuel pump
a) Disconnect the coil wire and crank the engine over for about 10 seconds to allow the fuel bowls to fill. This procedure will help prevent any back fires.
b) Reconnect the coil wire when finished.
For an electrical fuel pump
c) Let the fuel bowls fill in stages by turning the ignition key on and off, only letting the fuel pump run a few seconds at a time. This will help prevent the needle from seating improperly.
Step 2: Check The Fuel Pressure
Attach a fuel pressure gauge and check the fuel pressure. Flooding will happen when pressure exceeds 7.5 psi. According to Holley fuel techs, the fuel pressure should be no more than 7 psi; 6 psi is ideal.
Step 3: Use The Carburetor’s Sight Plugs To Inspect The Fuel Level:
With the engine off, remove the sight plugs from the fuel bowl to check the current fuel level. With the proper fuel pressure and float level setting, a slight trickle of fuel should be seen at the bottom of the sight plug hole when the vehicle is gently rocked in a side to side motion.
a) If no fuel is seen at the plug hole the fuel level is too low.
b) If fuel flows freely from the sight plug hole then the fuel level is too high. Anything other than trickle from the bottom of sight plug hole will require extra adjustment of the float to correct the fuel level in the bowl.
Step 4: Fuel Level Is Too Low:
If you didn’t see any fuel trickle out of the sight plug, the float is not allowing enough fuel to get into the fuel bowl.
a) With the engine off, raise the float assembly in the fuel bowl by loosening the lock screw on top of the fuel bowl. Make only a few turns and then gently pry up on the adjusting nut underneath to break the seal on the gasket.
b) Cinch the locking screw down tight again before starting the vehicle.
c) Start the engine and remove the sight plug.
d) Turn the lock screw counter clockwise with an 5/8-inch box wrench to loosen. (Vehicles with electric fuel pumps do not need to be started for fuel level adjustment.)
e) Wear safety glasses and cover the needle with a rag during the adjustment. The fuel will be under pressure and will most likely discharge at the needle when performing a float level adjustment on running vehicle. Wear safety glasses and cover the needle with a rag during the adjustment to prevent any fuel from coming into contact with hot engine parts.
TIP: You can poke a hole through a rag with the screwdriver and then place a 5/8-inch box wrench around the adjustment nut and let the rag drop down into place to absorb any spraying fuel during adjustment.
f) Turn the adjusting nut counter clockwise while holding the lock screw in place up top. This will let the fuel levels rise in the bowl. Keep turning until the fuel level reaches the bottom of the sight hole and starts to trickle out.
Step 5: Fuel Level Is Too High
If fuel is pouring out of the sight plug then the fuel level is set too high, and the float must be lowered in the fuel bowl.
a) Begin by lowering the fuel level in the fuel bowl to bring down the float. With the engine off, remove the sight plug and allow excess fuel to drain from the sight plug into a rag. Rock the vehicle a little to slosh additional fuel out of the bowl. Failure to lower the fuel level below the sight plug before adjustment will result in a bottomed out float in the floor of bowl.
b)Loosen the lock screw, and turn the adjustment nut clockwise for about 1/2 to 3/4 of a turn and then retighten the lock screw.
c) Start the engine to refill the fuel bowl and check to see the where the fuel level is in relation to the sight hole. Continue the proper procedures until the fuel level is at the bottom of the sight hole.
d) When the level looks correct, reinstall the sight plug and tighten it. On Holley models with sight windows, the fuel level should just appear in the bottom of the window.
Step 6: Finish By Tightening The Locking Screw
When you’ve made all your adjustments to the fuel float, make sure to tighten the locking screw while holding the adjusting nut with the box-end wrench.
Some Holley four-barrel carburetors, with square fuel bowls have no external float adjustment, but do have a sight plug to observe the fuel level. If you suspect a fuel level problem, follow the procedures in steps 1 through 3. A slight trickle of fuel should be seen at the threads. If not, do not disassemble the fuel bowls without first contacting a Holley technical service representative. For more information, contact Holley Performance, 1801 Russellville Rd., Bowling Green, Kentucky 42101. 270 782-2900. Additional Information can be found on Holley’s website at http://www.Holley.com.
Holley TV Video on Fuel Float Adjustments: