I get asked often how to tune a carburetor. Carburetors need occasional adjustment since they are manually tuned. Unlike Fuel injection on modern cars which is controlled by the onboard computer, carburetors depend on you to keep them in tune.
You should tune your carburetor in the summer, the winter, if you are driving in an altitude that has significantly changed, if you have modified the engine or added any parts that affect intake, exhaust, or timing. You should also adjust your carburetor if you change the timing on the distributor.
There are typically three types of carburetors:
- single barrel or monojet – When looking down on it from the top, this carburetor has only one ‘barrel’ and will have only one butterfly valve.
- two barrel – This carburetor will have two barrels when looking down from the top. You will also see two butterfly valves, one in each barrel.
- four barrel or quadrajet – A quadrajet is a type of four barrel, but does operate differently from a standard four barrel. Looking down from the top of either of these, you will see four barrels and four butterfly valves, one in each barrel.
Tuning any of these types of carburetors is pretty much the same. Near the base of the carburetor you will see one or two screws. One barrels have only one screw at the base. All other carburetors will have two screws. These screws are your mixture screws. They control how much fuel is let into the carburetor and mixed with air. If you have too much or to little fuel coming from your carburetor your engine will not run at it’s best. You will lose horse power, and have poor gas mileage. It’s important you tune your carburetor for optimum performance.
Here are the steps to tuning your carburetor:
- Start your engine and let it warm up
- Make sure your choke is open. If you have an electric choke it should be open at this point. If you have a manual choke, should check that it is open.
- While the engine is running, start turning one of the screws at the base of the carburetor in the direction that causes your engine RPM to increase. Keep turning the screw as the RPM increases. At some point the RPM will peak, and start going back down. Once this happens, turn the screw back the opposite direction so that the RPM is back at it’s peak.
- If you have two screws at your base (two barrels, four barrels, and quadrajets), you now adjust the second screw at the base of the carburetor. Do the exact same thing you did with the first screw. Once the RPM has peaked with the second screw stop turning it.
- Now it’s time to adjust your engine RPM. Chances are at this point your engine RPM may be too high at idle. There will be another screw, near where your throttle linkage connects to the carburetor. Turn this screw to bring the RPMS up or down to an idle that you are happy with. Generally I keep my idle RPM around 700 RPMS, and this would probably be alright for the motor you are working on too.
NOTE: If you have multiple carburetors, adjust the mixture screws for each carburetor as stated above. The only difference is that when adjusting the idle RPM, the linkage that connects all the carburetors together will have a way to adjust RPM so that it keeps all the carburetors in sync.
That’s it! Your carburetor is now tuned! Go for a test drive and make sure it feels good. You should notice your throttle will be more responsive, and generally runs better. If not, you may have other problems such as a timing issue, or possibly problems with your carburetor which would require a repair.