The simple science behind the art of carburation
TThe carburetor flickered out of existence in modern American automobiles in 1990, but nearly 30 years later, those of us with the inclination can walk into a motorcycle dealership and ride out on a brand-new carbureted machine. Despite the device’s longevity, it isn’t widely understood. To this day, when people talk of accelerating, they say, “I gave it the gas.” That’s a misnomer.
Whether our vehicle’s engine has modern digital fuel injection or carburetors, the action we take to accelerate is to open the throttle, which controls only the flow of air into our engine’s cylinders. Some other system then adds fuel in correct proportion to that airflow, resulting in a combustible air-fuel mixture being drawn into our engine. Before the digital-fuelinjection era, beginning in the late 1970s and completed in the 1990s, that system was a carburetor.
Not all mixtures of gasoline and air can be ignited by the hot spark that jumps across the electrodes of spark plugs. Only mixtures between 10 parts air to one part fuel and 18 parts air to one part fuel fit that bill. We want fuel to burn completely to get all the chemical energy it contains. That happens when all its carbon atoms join with oxygen from the air to form carbon dioxide, and all its hydrogen atoms combine with oxygen to form water. This occurs at a mixture of about 14 parts air to one part fuel—the so-called chemically correct mixture.
Delivering this correct mixture across the range of engine speeds and throttle openings from idle to maximum is no easy problem. The earliest carburetors were simple evaporators, which passed airflow across a wick kept wet with fuel. Because evaporation is a cooling process, wick carburetors had to be heated. The vehicle operator had to adjust how fast fuel was dripped onto the wick, and then correct the mixture by controlling an air bleed. It was difficult to get smooth, steady operation.
Wilhelm Maybach in Europe and Oscar Hedstrom at Indian in the U.S. had the idea of the spray carburetor. It uses Bernoulli’s Principle, which observes that when air moves, its pressure drops. The essence of their invention was to place one end of a small vertical tube in a cup of fuel, and then place the other end inside the pipe carrying engine airflow. Because pressure in the fast-moving
intake flow was less than pressure outside it, fuel was driven up the small tube to spray out of its end, mixing with engine airflow.
But Maybach and Hedstrom both found that the fuel-flow rate decreased as the gasoline level in the cup fell. The fuel had to be lifted farther up the pipe. That one was easy—just provide a float-controlled valve like the one that keeps a constant level of water in toilet tanks. As the level of fuel fell, the float dropped, opening the fuel valve and restoring the fuel level. The little cup and its float-controlled valve are the carburetor’s float bowl, which keeps the liquid fuel at a constant height.
There was another problem. The faster our engine runs or the more we open the throttle, the faster air moves through the intake pipe, and the air pressure drops. Why does air pressure fall as it moves faster through a pipe? In still air, all the motions of its molecules contribute to pressure by their constant random collisions. Air pressure is the sum of all those collisions. But when the air begins to move, some of its molecular motion becomes organized in the direction of the airflow, leaving less-random molecular motion to create pressure.
This loss of pressure as air moves faster and faster is also a loss of density. That means that the ratio of air to fuel changes, becoming steadily richer (containing more fuel in proportion to air) as intake airflow speeds up. All simple carburetors enrich as engine airflow moves faster through them. If uncorrected, this natural enriching tendency wastes fuel, and eventually the mixture becomes too rich to ignite.
A great many ingenious devices were invented to correct this natural enrichment—things such as spring-loaded air valves that let in extra air, or multiple fuel nozzles exposed in series as the air throttle opened—each one set leaner than
This is Keihin’s venerable CR, which has existed since the 1960s. As you see them here, four CRS have been “racked” into a unit, keeping them aligned, and greatly simplifying the task of making a singleor dual-throttle cable accurately control all four throttle slides. It was Gilera in Italy that first mounted carburetors this way. The CR, with its integral float bowl, was a huge step forward from the notalways-predictable fuel flow of carbs with separate float bowls.
the one before it. But the simple concept that worked best was to bleed some air into the fuel flow. The faster the fuel flowed, the more bleed air flowed along with it, and by sizing everything correctly, a constant ratio of fuel flow and airflow could be achieved.
If the operator suddenly opens the throttle to accelerate, the fuel is momentarily left behind because it is 640 times denser than air, resulting in momentary leanness. The engine either stumbles or cuts out completely. To prevent this, some carburetors included an acceleration pump, whose little piston, moving with the throttle, forcibly squirted fuel into the airflow to prevent momentary leanness.
Gasoline is a mixture of different hydrocarbons having a range of volatilities (ability to evaporate). At the temperature of a very cold engine, only about 10 percent of the fuel can evaporate, while the rest remains incombustible in liquid form. The result is a mixture too lean to fire. For cold starting, the mixture is enriched enough (by a choke, or starting carburetor) that there is enough volatile material evaporating to allow the engine to fire and start. As the engine warms up, its intake system becomes warmer, evaporating more and more of the fuel, allowing this temporary enrichment to be reduced until, with the engine at operating temperature, it is able to evaporate all the fuel flowing to it.
Carburetors at their best weren’t very good because, being passive devices, they could not compensate for changes in atmospheric density from weather, altitude, or humidity. They ran lean in winter and rich in summer—unless tuned for existing conditions by a human, adjusting fuel delivery by changing the sizes of fuel metering orifices, called jets. Today’s digital fuel systems are active, making such adjustments automatically. But carburetors are cheap and well-understood by the engineers who still employ them. They are also relatively simple and robust, and while they might not always operate at peak performance, they will function well enough to get a machine down the road even while far out of tune. They propelled the motoring world for nearly 100 years, the small devices that took a simple principle and carried it far into the future, and us along with them.
In 1977, Lectrons suddenly appeared on Kenny Roberts’ TZ750. Riders praised their part-throttle response and fine mixture formation. Yet they were a classic mass-production job, made of die castings assembled with self-tapping screws. The function was in the cylindrical needle, which instead of being tapered had one or more sloping flats ground onto its downstream side. With the needle located on the vacuum side of the slide, Lectrons had the strongest Venturi vacuum in the business. Transparent float bowls revealed the effects of carb vibration.