Finding the right air and fuel mixture will help extend the life of your engine. If your engine makes too much noise as if it were straining to run, it is important that you adjust the mixture and find the correct idle speed to reduce stress on the engine. That way you will prevent the engine from running too fast or too slow. You can adjust your car's carburetor in a few simple steps and without using special tools. Read the steps below to learn more.
Method 1 of 2: Adjust the Air Fuel Mixture
Step 1. Locate the air filter and remove it
On most cars, you will need to remove the air filter to expose it to the carburetor for adjustment. Before locating the air filter and removing the assembly, open the hood and make sure the engine is off. Unscrew the wing nut and any other connectors, and then remove the air filter completely.
- Depending on the make, model and engine type of the vehicle, the air filter can be located in several different places on the engine. Consult your vehicle's user manual or buying guide.
- In most carbureted cars, the air filter housing is attached directly to the carburetor.
Step 2. Find the set screws on the front of the carburetor
On the front of the carburetor, you will find two bolts that are used to adjust the fuel / air mixture.
- You can use a screwdriver to turn them because they generally look like flat head screws. That way, you can adjust the amount of fuel-air mixture in the carburetor.
- Some carburetors, like the Quadrajet (found in most GM vehicles), have a special screw and require a specific adjustment tool. The Quadrajet uses a double "D" carb setting tool.
- Other carburettors may have a 4 corner mix adjustment (4 mix screws).
Step 3. Start the engine and allow it to warm up to normal operating temperature
Check the temperature gauge to identify the appropriate temperature and listen to the sound of the engine so you know what adjustments to make.
- An engine that runs on a lean mixture at high revolutions per minute it will make a loud sound when opening the throttle, as if the mechanism is flooded. You need to add more gas to the mix.
- An engine that runs on a rich mixture it won't necessarily show a sound change, but you will be able to smell it. Decrease the gas a little. An engine that runs on a very rich mixture will cause the spark plugs to char, making it more difficult to start a cold car.
Step 4. Tighten both screws in the same way and find the correct mixture
Adjusting the carburetor is a lot like tuning a guitar or other stringed instrument. You must turn the screws in the same way, carefully and slowly until you find the ideal point. Regardless of whether the engine is running too lean or too rich, make the mix too lean by turning both screws a quarter at a time, counterclockwise, and then make the mix equal and homogeneous.
Adjusting the mix is an imprecise art where you need to know your engine well and listen carefully. Turn both screws slowly and listen until the engine purrs smoothly. If you hear a harsh sound or drumming, it means the mix is too lean. Keep turning the screws until you find the sweet spot
Step 5. Replace the air filter assembly
When you've adjusted the carburetor, put the air filter back in place and the car is ready to go.
If you also need to adjust the idle, wait to put the filter back in place until you're done
Method 2 of 2: Adjust Idle
Step 1. Locate the throttle cable and the idle adjusting screw that is attached to it
The cable starts at the throttle or gas pedal and runs through the fan housing to the carburetor. As always, check your owner's manual or buying guide for the make and model of your motor if you can't find the bolts.
Step 2. Start the engine and allow it to warm up to operating temperature
As you did with the air-fuel mixture, allow the engine to warm up a bit to make sure you adjust it to the actual operating condition.
Step 3. Turn the idle adjusting screw to adjust it
Turn the screw clockwise, no more than half, and listen to the motor. Most user manuals indicate the optimum speed at which you should adjust the idle, although you can make some necessary changes if you prefer it to be higher or lower. Check your owner's manual for the speed number and check the tachometer as you make adjustments.
Step 4. Listen to the engine for straining sounds and readjust if necessary
It should take about 30 seconds for the motor to adjust to the change you made, so don't rush and over-tune it. Turn the screws slowly and listen to the sound carefully.
Step 5. Replace the air filter and finish the job
When you have adjusted the idle to the appropriate specifications or your personal preference, turn off the engine and replace the air filter to complete the job.
- The idle speed increases if you tighten the screw. On the other hand, if you loosen it, the opposite happens.
- If your car is equipped with a tachometer, you can use this device as a tool to adjust idle (revolutions per minute or rpm). Check the owner's manual of the car to know the r.p.m. correct.
- If the engine does not run smoothly after adjusting the idle mechanism, re-adjust the air and fuel. Also, repeat the steps for air, fuel, and idle adjustments.
- There is a difference in idle speed with the car parked and an automatic car running. Do not adjust idle speed with the car running unless someone is in the driver's seat with their foot on the brake.
- There are some cars that have multiple carburettors, such as the European and Datsun, whose airflow must be balanced before adjusting.
- Remember that when you work with a carburetor you also work with a fuel source. Take all necessary safety measures when working around gasoline.
- Keep in mind that you are also working with an engine running. You can seriously injure yourself or damage the engine if you are not careful. Work slowly and carefully and make sure your clothes don't have dangling strings or cords that could get caught in the motor.