A carburetor is a mechanical device that blends fuel and air in an internal combustion engine. Although modern vehicles largely don’t use carburetors anymore, relying instead on fuel injection engines, there are still quite a few older vehicles on the road that have them. Carburetor issues can cost several hundred dollars to fix and affect the performance of your vehicle significantly, so you need to be able to tell for sure when an overhaul is in order.
When a carburetor is working properly, it provides a mixture that has just the right ratio of fuel to air. Combustion thus is very efficient and much more safe, occurring at a temperature that is neither too hot nor too cold. The carburetor also helps control the speed of your engine. Understanding these basic principles, carburetors that are damaged, dirty or maladjusted can result in issues such as hard starts, poor acceleration, stalling, fast or rough idling, hesitations/misfires and poor fuel economy. Other problems can cause these symptoms, too, however. For example, a plugged fuel filter can result in a decrease in the amount of fuel that’s able to flow from the fuel pump, reducing power and potentially resulting in stalling or misfiring. Subsequently, you shouldn’t automatically assume the carburetor needs work. You need to inspect the carburetor and perform some tests to diagnose where the root of the symptoms really is.
Tests and Checks
A choke is a device that restricts how much air can flow into the entrance of the carburetor, which subsequently creates extra vacuum in the carburetor chamber and pulls in extra fuel. Depending on whether the choke won’t open or can’t close, the fuel mixture can be either too rich (revealed by black, dirty electrodes on spark plugs) or too lean (revealed by a yellowed or blistered insulator around the center electrode), which results in quick idling, difficulty starting the vehicle or stalling. Check that the choke is properly adjusted, that bi-metal heat-sensing spring that opens the choke is intact, that the heating element that warms the spring has power and that the choke linkage and shaft are not sticking due to residue or dirt.
If you are experiencing hesitation/misfiring when you accelerate, the accelerator pump could be malfunctioning. The rubber components can become worn or torn, the discharge nozzles can become plugged or dirt can get into the fuel bowl. The steel check ball can become stuck, as well, creating what amounts to a pressure leak that stops the pump from squirting fuel. If the hesitation only happens under load, the power valve inside the carburetor might be clogged or have a bad diaphragm, which requires replacing the valve.
When fuel economy is poor, the problem is often with the carburetor float, which determines the fuel level in the bowl. Adjust the float setting or weigh the float to determine if it’s become too saturated with fuel to work right. Poor fuel economy also can relate to a dead oxygen or coolant sensor, as well as to improperly calibrated jets.