That Frightening Thing Called a Carburetor
| 31 May 2007
Do you . . . . would you . . . . service your own carburetor? If you look at an exploded view of a typical Ford 2-barrel carburetor, it looks frighteningly complex. The picture that accompanies this pony trick shows a very simple carburetor to give you a flavor of how the device functions; but if you go online to http://www.carburetorfactory.com/expvw07.html, you can look at a real Ford unit.
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Despite the complex appearance of the Ford carburetor, if you had one like it that needed work (or a 1-barrel or 4-barrel, for that matter), you could probably bring it up to the factory performance levels if you thoroughly cleaned it, installed a rebuild kit, and made some adjustments. In addition to the kit, you would need a tachometer, possibly a vacuum gauge, a 6-inch engineer’s ruler graduated in 1/64” increments, a few simple tools, and an aerosol can of Gumout Choke and Carburetor cleaner.
Instructions for installing the kit come with the package and factory adjustments are available in service manuals. Assuming you removed all of the visible dirt and sediment from the inside of the carb and sprayed cleaner through all of the passages to be sure they weren’t blocked, it should perform well when you put it back together with new parts and gaskets and then adjusted it properly. Convincing yourself that you could actually do the job might be the hardest part of the project. But weighed against the cost of a refurbished unit from someone like Pony Carburetors, maybe you would want to try.
We all know that Pony Carburetors and others do an excellent job; these shops are certainly worth serious consideration for a concours restoration where appearance is at least as important as performance. But for a daily driver where the goal is reliable performance, a carburetor serviced by the owner could be just as good. Anyway, it’s food for thought.
Contrary to what you probably expected after reading the lead-in, this Pony Trick is not a “how-to” article on carburetor rebuild procedures. Rather, it looks at some carburetor history and theory. With a little background in your pocket, tackling a rebuild might seem like a reasonable path to follow. Because, as said, with a rebuild, all you’re doing is cleaning (meticulously), replacing parts, and making adjustments.
Touching briefly on history, where does the name “carburetor” come from? The base word is “carbo”, Latin for carbon. And “carburet” derives from the French word “carbure”, the process of combining carbon-containing chemicals with a gas. A carburetor, then, is the device used to combine a carbon-containing chemical (the hydrocarbon called gasoline, in this case) with air (the gas we breath). Hungarian engineer Donát Bánki is credited with inventing the carburetor in 1893.
There are three generic carburetor designs, each named for the direction that air flows through it. The updraft design finds the air column flowing in an upward direction and picking up gasoline along the way. The side-draft device has the air column flowing in from the side and finally, the air column flows downward in the downdraft design. All three designs work equally well; the choice of one over the other is frequently related to packaging.
The updraft design is usually found on older engines, those built before paper air filters were available. The early air filters were an oil bath design where it was necessary to keep the oil from migrating toward the engine as air flowed through the filter and into the carburetor. With air flowing upward , the oil stayed in the filter. A side benefit of the updraft design was it’s inherent ability to help prevent flooding when the engine was being started. If the mixture was too rich (too much gasoline in the air), the excess gasoline would fall out of the carburetor away from the engine. The side-draft carburetor was popularized in Britain where underhood room was limited on some cars while the downdraft unit was popularized in the US with the proliferation of the V-design engines (e.g.: V-8) where the device typically mounted between the engine banks.
Backing up to the definition of a carburetor as a device that mixes gasoline with air, the natural question becomes: how does it do its job? In simplest terms, a carburetor responds to various vacuum signals and mechanical adjustments in a way that controls the amount of gasoline introduced into the air column flowing through it. The air column exists because the engine to which the carburetor is attached is a big vacuum pump and, as such, it looks for air to replace the negative pressure it creates. The gasoline used by the carburetor comes from a reservoir inside the carburetor housing. The reservoir is called the float chamber or fuel bowl. As gasoline is consumed from the float chamber, it’s replenished by the fuel pump. The amount of gasoline in the bowl is controlled by a float that closes a valve when the bowl is full. “Full” is a relative term meaning when a specified amount of gasoline is present. An adjustment called float level is used to meet the specification.
Since air is the medium that carries the fuel, the primary carburetor control—called a throttle plate or throttle valve or butterfly valve—is designed to regulate the amount of air that’s allowed to flow. When the air is controlled, it then becomes a matter of providing the correct amount of fuel for various operating conditions.
The throttle plate is a moveable baffle between the carburetor and the intake manifold. It’s mounted to a shaft in the outlet side of the carburetor. The shaft in turn is connected to the linkage that leads to the accelerator peddle. When the accelerator is depressed, the plate opens proportional to how far the peddle is moved: fully open when the peddle is floored, closed when the peddle isn’t depressed at all, and infinite positions between the two extremes.
With the engine running and the throttle closed, the carburetor delivers gasoline to the air column through what’s called the idle circuit. The closed butterfly causes a significant vacuum to develop. The vacuum is sufficient to pull fuel through a small port placed after the throttle valve but only a small amount of air and fuel is provided under these conditions. The small volume of mixture can generate so little combustion force that mechanical intervention is needed to make sure enough of it is delivered to keep the engine running. Mechanical control is provided by adjusting a screw that holds the “closed” throttle open a tiny fraction to allow a little more air to pass. This adjustment is called idle speed and it’s set to specification by using a tachometer to report on engine RPM. Another screw, called a needle valve, serves as a valve in the idle circuit fuel passage. At a given idle speed, the mixture screw causes the engine to speed up or slow down when it is turned in or out. Speed changes because the amount of gasoline allowed to pass depends on the position of the adjustment mixture screw. Not surprisingly, this is called the mixture adjustment. Again the tachometer is used to read RPM with the correct setting being 1/4 turn clockwise (lean) from the peak RPM observed as the screw is rotated slowly back and forth. The reason for going 1/4 turn lean is to reduce emissions.
These adjustments interact with each other so that the optimum idle adjustments boil down to finding the highest vacuum at the correct idle speed. A vacuum gauge can be used in conjunction with the tachometer to pinpoint the settings but usually peak vacuum develops at the mixture screw position where maximum RPM is seen. After finding the peak and then closing the needle valve 1/4 turn, it may be necessary to set the idle speed again because of the interaction.
As the accelerator is depressed and the butterfly is opened from the idle position, an increasing volume of air passes but vacuum begins to drop. With vacuum falling, the idle circuit can no longer provide enough gasoline to the air column. This is a transition phase where fuel requirements are supplemented by the off-idle circuit. When the throttle opens slightly from the fully closed position, it swings past another fuel delivery port located slightly higher in the carburetor throat. The port allows more fuel to flow, helping to compensate for reduced flow from the idle fuel port.
As the throttle continues to open, the vacuum signal drops even further, the result of less restriction to airflow. Lower vacuum means reduced gasoline-flow through the idle and off-idle circuits, just when more fuel is needed. But there’s one more circuit called the main circuit. As the opening throttle plate allows more and more air to pass, the main circuit comes into play, eventually replacing fuel contributions from both the idle and off-idle circuits.
Main circuit operation depends on the carburetor throat being shaped like a venturi, named after Italian physicist Giovanni Battista Venturi; i.e., the throat has a constriction in it. When air passes through the venturi, it speeds up at that location. The increase in velocity creates a partial vacuum as defined by a theory called Bernoulli’s principle, named after Dutch/Swiss mathematician/scientist Daniel Bernoulli. The end result is that increased air speed causes the partial vacuum in the venturi to rise high enough to suck gasoline through a nozzle and into the air column; the nozzle is located in the venturi. The fuel mixes with the air column and becomes the combustible mix supplied by the carburetor under operating (driving) conditions.
With sedate acceleration and relatively steady driving, the three circuits allow the engine to perform well. But sudden opening of the throttle butterfly or wide-open-throttle (WOT) operation both require help from additional carburetor circuits, each designed to rapidly increase fuel flow on demand. The circuits are fed by devices within the carburetor. One device is called the accelerator pump and the other is the power valve.
When the butterfly valve is opened suddenly, whether it be sudden acceleration from idle or sudden acceleration for another reason (e.g.:engaging passing gear in an automatic transmission), more fuel is required than can be provided to an air column that has suddenly seen a major drop in vacuum. Using a so called accelerator pump mechanically linked to the throttle plate mechanism, one shot of fuel is pumped through a nozzle into the carburetor throat. The shot is delivered at the same time the butterfly is snapped open. The delivery of extra fuel sustains operation until venturi vacuum can take over again. A faulty accelerator pump may cause the engine to stumble when the shot of fuel is poorly delivered or not delivered at all.
If the engine is being operated at WOT, more fuel is required than normal carburetor operation can supply. The accelerator pump provided a shot when the throttle was opened but sustained WOT operation requires a constant supplement. The power valve opens at that point and remains open as long as needed. The valve is controlled by vacuum and a spring. It remains closed under normal driving, held in that position by vacuum. But being spring loaded, it’s designed to open when spring pressure is greater than vacuum suction. At WOT, vacuum is very low so the spring opens the valve. When the throttle is closed far enough for vacuum to overpower the spring, the valve closes. Without the power valve, the engine may run lean at WOT (too little gasoline in the air column). A lean mixture, especially when the engine is working hard, may cause serious engine damage from pre-ignition and overheating. Sometimes the power valve will fail causing constant over rich operation under normal driving conditions; fuel consumption will increase dramatically, the engine will run rough, and there will be a telltale black trail from the tail pipe. Many Ford power valves use a diaphragm design contained in a housing mounted to the outside of the carburetor. When those valves fail, the engine not only runs rich under normal conditions but there is also a gasoline leak through the pressure-equalizing hole in the housing.
A lot more could be written about the carburetor but I’ll end with some comments about the choke, in this case an automatic choke heated by hot air. There are chokes heated by electricity and also manually operated chokes that aren’t heated at all. Usually electric and manual chokes are associated with older engines (manual type) or modified engines with non-stock carburetors (electric type).
When the engine is cold, gasoline doesn’t vaporize very well so it tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start. To work around this situation, a richer mixture is required to start and run the engine until it warms up. To provide the extra fuel, a choke is typically used to block some of the air from entering the carburetor. The choke is a baffle that resides at the entrance to the carburetor throat. With this restriction in place, extra vacuum is developed in the carburetor barrel. The relatively high vacuum causes fuel to be delivered through the venturi nozzle to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture needed to start the engine and sustain operation at low engine temperatures. Usually, a cold engine is also idled at a higher RPM to aid the venturi effect. But as the engine warms up, the rich mixture needs to progressively lean out to normal operating level. To accommodate this, the choke is mounted on a shaft so that it can open to allow more air to flow and a progressively leaner mixture to develop as operating temperature increases.
A bimetallic spring closes the choke when it’s cold and as the spring warms up, it unwinds in a motion that progressively opens the choke butterfly. The spring is warmed by hot air from a stove built into, or mounted on, the exhaust manifold or header. Vacuum from the carburetor sucks the hot air through a tube into the housing where the spring is mounted. To help open the choke, vacuum also pulls on a small vacuum motor in the housing. Normally the choke will be fully open in about 7 or 8 minutes, a time period that closely tracks the time it takes the engine to warm enough to run at idle without help. When the choke reaches the full-open position, it’s held there by continuing to supply hot air to the hot bimetallic spring and by the vacuum signal that helped to pull it open.
Fuel Injection Challenge:
I’ve serviced carburetors for years and feel comfortable doing so. But other than a basic understanding of fuel injection and its benefits, I’m not up to speed on the fine points of a computer controlled injection system.
I’d like to challenge anyone fully familiar with fuel injection to write a pony trick about how it works and how the engine management computer uses input/output signals to control it.