While modern cars use electronic fuel injection (EFI) systems, most pre-1990 vehicles — and modern power equipment and motorcycles — still use a carburetor to get fuel into the engine. It’s a simple system and highly reliable, but not accurate enough for modern emissions standards, which is why it was replaced by EFI. When diagnosing fuel problems in a carburetor vehicle, it’s essential to understand the role of the various parts, such as the carburetor float, venturi, throttle valve, jets and others.
How a Carburetor Works
At its most basic, a carburetor is a tube of fuel in the air stream before the throttle plate. A narrow section, the venturi, increases local air flow, reducing pressure. This reduced-pressure area pulls fuel through the jet into the air stream, mixing and evaporating it on its way through the intake manifold and into the cylinders. Fuel flow through the jet is controlled by a needle, tuned to improve fuel economy and performance.
Mounted on the side of the carburetor is the float-feed chamber, or “bowl,” which is essentially a miniature fuel tank fed by the main fuel tank. Because the carburetor can’t use fuel under pressure, whether delivered by a fuel pump or gravity, the float chamber is kept at atmospheric pressure. The carburetor float needle, moved by the float, controls fuel flow, maintaining the fuel level in the chamber.
As suggested by its name, the “float” needs to float in the fuel, so it’s usually made of hollow plastic, metal or fuel-resistant foam — some were made of cork in the old days. As the fuel level drops in the float chamber, the float drops with it, opening the float needle and allowing fuel to enter the float chamber. As the float chamber fills, the float moves up, closing the float needle and stopping the flow of fuel into the chamber.
Common Carburetor Float Problems
- Engine flooding — this is by far the most common problem with carburetor floats. If the float sinks, the float needle stays open, filling the float chamber to the top and then forcing fuel into the carburetor, flooding the engine. This can be caused by metal floats corroding or plastic floats cracking and filling with fuel. The float could also break off, causing the same problem, but this isn’t common.
- Running too rich or too lean — On some carburetors, the float is adjustable, usually by a screw or small metal tab. If the carburetor float is too high or too low, this can skew fuel trim too high or too low. Saturated foam floats are often to blame for rich-running problems. You may be able to adjust the float level with a screw or by bending the tab.
- Stalling at high speed — this may be due to a carburetor float that’s adjusted too low and doesn’t keep enough fuel in the chamber. At high speed, the carburetor is pulling so much fuel out of the chamber that the fuel pump can’t keep up. If this happens a lot, you could have a fuel delivery problem, such as a stuffed fuel filter or kinked fuel line, or you might need a different carburetor or fuel pump setup. You may also be suffering the effects of ethanol degrading your fuel system, which can me prevented by using a fuel conditioner.
Although carburetors have become obsolete in a modern emissions world, you can still find them everywhere — probably even in your own garage. Taking care of a carburetor (or even rebuilding one) doesn’t require more than basic hand tools and cleaning supplies. You can also help keep the inside of a carburetor clean by using a motor treatment like Sea Foam on a periodic basis.
Check out all the Fuel & Emissions System products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on what a carburetor float does and common problems associated with it, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Wikimedia Commons.
Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.