How to Tune a Four-Barrel Carburetorby Don Bowman
Tuning a four-barrel carburetor for the highest performance requires jetting as well as adjustments. On a four-barrel carburetor, there is a primary side and a secondary side. Both sides are jetted differently. The primary side is the most used and has the idle adjustment that feeds fuel from below the throttle plate. The main metering jets are located above the throttle plate and require a lot more vacuum to operate. The vacuum is produced through the air passing the ports causing a low-pressure area. As the volume of air or CFM increases, the vacuum correspondingly increases and the fuel flow does the same. The carburetor must be totally clean and all passages open to be tuned properly.
Idle is irregular, rough or too low
Adjust the idle mixture screws in the front bottom of the carburetor. Starting with the driver's side, screw and turn the idle mixture screw in until the RPM begins to drop.
Turn the screw back out until it reaches its maximum RPM and is no longer climbing. Do the same for the passenger side screw. Repeat the process one more time, making sure that the screw is only turned out enough to reach maximum RPM.
Adjust the idle screw at the throttle linkage and set the idle at about 650 RPM at idle and in park.
Engine has a dead spot when accelerating or hesitation:
Adjust the accelerator pump. All engines have a drop in vacuum when the throttle plates are suddenly opened. If a vacuum gauge were observed while idling, the vacuum should be very close to a steady 18 inches of vacuum. However, if the throttle is suddenly opened the vacuum momentarily drops to 0 and then starts to climb as the RPM increases. This sudden drop in vacuum shuts off all fuel flow momentarily and that is where the accelerator pump comes in. It is a cylindrical chamber in the float bowl and has a plunger attached to the throttle and pump raw fuel into the engine every time the throttle is opened. This takes up the slack when the vacuum drops because it is not dependent on vacuum. It is either out of adjustment or malfunctioning when a dead spot appears.
Remove the air cleaner and look into the top of the carburetor. There is always a nozzle located in the front primary up high in the bore. There are many large performance carburetors that have two accelerator pumps; one in the front and one in the back. These are referred to as "double pumpers."
Open the throttle with one hand while looking in the top of the carburetor. A large stream of fuel should be seen coming out the nozzle every time the throttle is opened and as soon as it is opened. If the throttle must be moved a ways before the fuel flows, then the rod must be adjusted so that the pump reacts sooner. If no fuel comes out, the pump has a bad rubber diaphragm and must be replaced.
Engine runs rough at idle and runs out of fuel when accelerating
Remove the air cleaner. Look into the top of the carburetor while the car is idling. If fuel is seen dripping, the float is stuck or needle and seat are dirty and stuck. On a Holley carburetor, the needle and seat are on the top of each float bowl and can be removed and cleaned. Just remove the top screw and turn the nut out and it will withdraw the needle and seat. When installing it, run it in until about three threads are showing under the nut and install the screw.
Remove the sight plug on the side of the float bowl and adjust the needle up or down until fuel can be seen in the sight hole. It is to high if fuel comes out so turn the needle in until it is at the hole. On a Quadrajet, Formoco, Edlebrock or Carter AFB, the top of the carburetor must come off and the fuel level can be seen. The float arm where it contacts the needle must be bent to adjust the level.
Adjust the float level if the engine seems to be running out of fuel--the fuel level in the bowl is to low. Use the same procedure in Step 2.
Engine is erratic and bucks at a steady 3000 rpm:
Remove a spark plug and look at it for a lean (white or gray ashy burnt look) condition or a rich condition (black, sooty look). The most probable cause is dirty carburetor or the primary metering jets are a smidgen to small.
Increase the metering jet size by two sizes first and retry. If it's too much, switch out the jets to one size larger than the original jets to correct an erratic, bucking, lean mixture.
Decrease the metering jet size by one size for a rough running engine. The engine will idle roughly at a steady RPM, but will have black smoke and sooty plugs.
Engine does not accelerate well when the secondaries are opened
Check the secondary vacuum diaphragm for leaks.
Replace the spring in the vacuum diaphragm. Go to a one size weaker spring in the vacuum diaphragm to open the secondaries quicker.
On a Holley carburetor, make sure that the fuel level is correct in the rear float bowl before changing the spring in the vacuum diaphragm.
Things You'll Need
- Phillips screwdriver
- Small common screwdriver
- Small pair of needle nose pliers
- Can of carburetor cleaner
- Standard pair of needle nose pliers
- Standard size common screwdriver
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).