How to Tune a Carburetor Video – Part 3 Quadrajet Rebuild Series
We get down to basics on how to tune a carburetor. We’re down to the final turn on this 3-part series for rebuilding a Rochester Quadrajet Carburetor. This is definitely the simplest part, but probably one of the most overlooked for anyone still owning a carburetor.
Disassembly – Part 1 on How to Rebuild a Rochester Quadrajet Carburetor
Assembly – Part 2 on How to Rebuild a Carburetor
Steps on How To Tune a Carburetor
Specifically, we’re talking about how to tune a Quadrajet carburetor, but really, these steps can be applied to most carbs, including the Holley. There are really only 3 steps that need to be addressed. Of course, this is assuming that your rebuild is solid and you have no intake manifold issues.
Step 1 – Choke
The first step on how to tune a carburetor is dialing in the choke. In addition to helping with cold cranking, the choke also restricts or allows operation of the Secondaries (4-barrel). If you’ve ever Not heard that 4bbl howl when you plant your foot on the floor, it could be the choke holding the secondaries closed.
Loosen the 3 small straight-slot screws on the plastic choke cover, then rotate the cover until the choke blade is barely open (almost closed). This is assuming the Quadrajet is cold. Once the engine is up to temperature (160 degrees F), the choke blade should be straight vertical, and the secondaries should be free from bondage. If needed, loosen the 3 screws again, and adjust rich/lean until the choke blade is vertical.
Step 2 – Idle Air/Fuel Mixture
Using a vacuum gauge, adjust the idle air/fuel mixture screws on each side until the maximum vacuum is reached. Find a manifold vacuum port on the Quadrajet, such as the PCV port, and connect your vacuum gauge. With the car running at operating temperature, use a screwdriver to adjust the idle mixture screws. Start by turning in (clockwise) the screws until they bottom-out. Then, turn each idle mixture screw 3 or 3-1/2 turns out (counterclockwise). This will ensure that both screws are initially adjusted the same.
Start by adjusting either idle mixture screw, but don’t change until you’re completed with that screw. First, try to turn the screw 1/4-turn In (clockwise). If the vacuum increases, then turn another 1/4-turn in. If it decreases, go the other way. Find maximum vacuum, then move to the other screw and repeat the steps. Once the maximum vacuum is reached for both screws, you’re ready for the final step.
Final Step on How to Tune a Carburetor – Fast Idle
Finally, we just have to adjust the fast-idle screw. This screw has a spring on it and is located at the linkage, where it connected to your throttle cable or arm. You probably want to set the idle somewhere between 750 and 950 RPM. Turn the screw in (clockwise) to increase RPM or out to decrease RPM.
We purchased our Quadrajet Rebuild Kit from Mikes Carburetor Parts.