Carburetors 101: Mostly Reliable

While fuel injection is the modern norm on a wide variety of engines, it’s amazing that many common aircraft engines still have carburetors. We’re talking tech that goes back at least 100 years. While some can be finicky, in reality aircraft carbs are generally reliable with the right upkeep. But this isn’t to say they don’t have issues that can ruin an otherwise good flying day, as was the case last winter when I heard a Lycoming-powered Cherokee piloted by a low-time student spitting and sputtering in the traffic pattern. All’s well that ended well, but the idea is to keep carburetors performing at their best ahead of the emergency. Here’s an overview.

Mixing It up To Make Hydrocarbons

A short history lesson is in order, where the carburetor increases the amount of carbon in the air by combining it with gasoline—a hydrocarbon. Carburer, if you will, is a French term that refers to fueling an engine with gasoline. And in doing that, a float and needle valve long ago turned out to be the best way to keep a gasoline engine running at variable power settings. It’s also about the combustive charge of fuel and oxygen, otherwise known as fuel metering.

The majority of carburetors used in our common aircraft engines are the float type, which has a bowl that fills with fuel (via gravity feed or from a fuel pump) to a level regulated by a float attached to a needle valve. Fuel enters the bowl through a strainer that filters the fuel and as the fuel level increases, the float rises and the needle valve (attached to the float by a lever) closes and shuts off the fuel flow until the float level drops again. Air goes into the carb and travels through a venturi, which accelerates the airflow and causes the pressure of the air to drop. A nozzle is placed in this area of low pressure and is connected to the fuel bowl. The low pressure creates suction on the nozzle and fuel is discharged into the airstream. As the fuel is discharged it vaporizes.

The amount of suction on the nozzle is regulated by the mass airflow going across the nozzle, and the amount of airflow is controlled by the throttle or butterfly valve located downstream of the venturi and discharge nozzle. As the throttle valve is closed (by moving the cable-actuated throttle control), the airflow is reduced. Push the throttle in and the butterfly valve opens, and airflow and suction on the discharge nozzle are increased. When the throttle cable is pushed in all the way, the butterfly valve is wide open, known as a wide-open throttle setting.

There’s more. The mixture control on the carburetor controls the amount of fuel coming out of the discharge nozzle. The throttle controls the amount of suction, but the mixture controls the amount of fuel and allows for manual adjustment of the fuel-to-air ratio via the cable-actuated mixture control.

You often hear it’s running “too rich or too lean,” so what’s the right mixture? Ideally it’s 15-to-1—stoichiometric—where all the fuel and all of the oxygen in the air will be completely used in the combustion process. This produces the highest combustion temperatures because the proportion of heat released to a mass of air and fuel is the greatest. On our engines we generally use mixtures around 15-to-1.

But even when perfectly tuned, carbs don’t exactly work as seamlessly as a healthy fuel-injection system. When the throttle is opened quickly with a carburetor, the airflow increases suddenly and there is a slight delay in the suction on the nozzle, causing a lean mixture and engine hesitation or stumbling. In order to compensate for this, some carburetors use an accelerator pump, which is really nothing more than a plunger that enriches the mixture by injecting extra fuel into the airstream when the throttle is advanced rapidly. Moreover, some carbs have an economizer system that provides a richer mixture to the engine at high power settings, primarily for cooling and to prevent detonation. 

Trouble With Carbs

Perhaps the most common issue might be fuel-related, especially with fuel containing ethanol. It’s just not a good match for carburetors on engines that sit for long periods because of corrosion and the clogging of jets. If you have an engine approved to run on high-octane pump fuel, the strong suggestion is to fly it as often as you can.

For carbureted engines that run only on avgas, problems with the idling system (and water in the fuel) might be an issue. Consider that when the throttle is pulled back to idle, the air velocity through the venturi is so low that it cannot pull fuel from the float chamber via the fuel nozzle. That means low pressure exists on the engine side of the throttle valve, so a fuel passageway runs from the float chamber to a point near the edge of the throttle valve. The opening is the idling jet, and a separate air bleed system is included as part of the idling system.

Where do you start when trying to troubleshoot a carb problem? Methodically. You can help your shop cut to the chase by being specific about which conditions are causing problems. Does the engine only run rough or stumble on idle? Does it idle smoothly but stumble when applying power? Or maybe it keeps running for a while when you pull the mixture to cutoff.

Even when the carb is tuned properly, and like everything else on the aircraft, corrosion can wreak havoc on a carburetor especially when it comes to water contamination. The mounting clip for the floats, as one example, is often made of steel and can rust if exposed to water, as can the spring arm on the mixture control metering sleeve. Even the accelerator pump plunger might have steel components that can rust. Again, fly it often, and avoid water in the fuel at any cost.

Stay Ahead of Them

The best thing you can do to keep carbs running right is preventive maintenance and regular inspections. Perhaps the most common carburetor for common GA engines comes from longtime manufacturer Marvel-Schebler, a North Carolina company that dates back to 1905. The company is an OEM supplier and it has a substantial overhaul and rebuild business, primarily through a network of distributors. When we talked with the company last year, we were told the majority of carb problems on aircraft engines are much like on any engine: lack of use and improper upkeep. Marvel-Schebler’s service bulletins call for carburetor overhaul at engine overhaul or 10 years in service, whichever comes first. A common issue is gaskets and O-rings drying, and as we noted, if the airplane is parked outside it’s not unusual to get water in the fuel system, which can lead to corrosion inside the carburetor. You’ll pay close to $1,000 for an overhaul (which brings a carb to current specs) on most common models and twice that or more for a new replacement. Since it’s the manufacturer, it can also perform carb rebuilds using factory-new parts—which is one step above an overhaul.

Techs tells us that with the proper upkeep (keeping fuel filters clean, if equipped) and being mindful of the fuel you use, there’s no reason why carburetors can’t last to and beyond engine TBO. In between, pay attention to telltale symptoms that something might be wrong. This includes fuel leaks (again, dry gaskets), plus problems with the idle mixture and even leaks in the fuel primer system. A simple test: When shutting the engine down by pulling the mixture control, watch for a slight increase in engine RPM, which should be between 25 and 50 rpm. And no, when you pull the carburetor heat on at idle power the engine shouldn’t quit. On many engines, the magic healthy rpm at idle might be around 600 to 700 rpm, but follow the guidance in the POH. Like any other system, if something doesn’t feel right, it probably isn’t.