Text by Sam Logan.
Pictures by Moore Good Ink.
Engines produce vacuum, and over the past 130 years engineers have contrived ingenious ways to advance the carburetor’s powers to match engine developments.
Aided by barometric pressure, ignition and compression, the carburetor creates the air-fuel mixture that promotes combustion. What’s more, it mixes gasoline with air in the correct ratio for combusting at varying engine loads, engine and air temperatures and altitudes. Carburetors work by pressure differential; high pressure flows toward areas of low pressure. Through a labyrinth of small-bore drillings in the 4150-style four-barrel carburetor, the vacuum draws a potent mixture of air and fuel. So formidable is the mixture, the carburetor has empowered naturally aspirated full-bodied 500 cubic inch drag racing cars to speeds in excess of 213mph in a distance no greater than 1,320 feet!
On starting and at idle, the air speed is too slow to draw fuel from the carburetor’s main jets and through its boost venturii. So idle fuel is drawn from a low pressure area under the carburetor throttle plates, which at idle are almost closed (see illustration No. 3 below). As the engine gains speed, larger throttle openings provide sufficient air flow and the area of lowest pressure switches from the idle discharge ports to the boost venturii (see first illustration), which activates fuel flow through the carburetor’s main jets.
On 4150-style carburetors, as displayed in these illustrations, the boost venturii reside within the main venturii and low pressure (partial vacuum) is created by the constricting shape of their bores—the bores’ narrowest part—which causes air speed to increase and, as a result, a siphoning effect is introduced at the boost venturii’ tiny air-fuel discharge orifices and through the air-fuel metering circuits back to the fuel bowls. The faster the air speed in the venturii, the more fuel is withdrawn from the bowls, thus unleashing optimum engine power. As the fuel bowls are vented to barometric pressure—an area of greater positive pressure than that of the discharge orifice in the venturii—the air-fuel mixture is compelled to travel from the bowls to the boost venturii of the running engine.
The pressure difference at the venturii is activated by engine’s intake strokes; that is, as the engine’s pistons travel downwards on their induction strokes (and with their respective intake valves opening and closing), vacuum is created in a progression of gulps, drawing the air-fuel mixture from the carburetor through the intake tracts and into the cylinders.
However, on powerful engines at high speed or under high load, main jets cannot provide sufficient fuel without adversely affecting the carburetor’s performance at lower engine speeds, and power valves were introduced to resolve this difficulty. Vacuum operated and located on the carburetor’s metering block and residing between and above the main jets, they have direct access to the fuel bowl, see illustration No. 8 below.
To make the phase changes from liquid to vapor, fuel needs to be emulsified (mixed with air), atomized (separated into fine particles), vaporized (transformed to a gaseous state), and compressed in order for it to produce energy. The carburetor takes responsibility for the emulsification and the atomization processes while the vaporization occurs on the hot surfaces from combustion. Complete combustion and best fuel economy requires an air-fuel mixture of almost 15:1. Best power is achieved around 12.5:1.
When a cold engine fires and runs, combustion heat warms its parts quickly, enabling evaporation of the emulsified and atomized fuel from the carburetor. But when starting a cold engine, where a moving film of fuel clings to the walls of the induction tracts (leaning the mixture), there are no hot surfaces to vaporize the fuel and choking is required to enrich the mixture. In addition to heat, vaporization is also triggered by air speed, pressure, and surface area.
The carburetor’s choke mechanism (a mechanical valve) restricts the flow of air, and the ratio of this highly enriched starting mixture is reported to be as concentrated as one part of fuel to one part of air (1:1). Fortunately, gasoline produces dense vapor at a temperature lower than most liquids—140 degrees Fahrenheit—and the small proportion of fuel that does evaporate induces a mixture sufficiently rich for a spark to ignite in a cold engine. Once warm, opening the choke valve restores the carburetor to normal operation.
Tuning information with illustrations and captions are as follows. We identify most of the chief components of the 4150-type four-barrel carburetor, illustrate their functions, acquaint you with common problems that adversely affect them, and advise you how to resolve them.
But first, start by checking initial ignition timing, as it is frequently insufficient. Use 18 degrees BTDC as a starting point. If 18 degrees results in excessive total timing, retard it until the engine runs happily. Excessive total timing can be recognized by spark-knock under acceleration. Spark knock can also be caused by excessive compression ratio.
Lastly, in this area, in the Atlanta suburb of Doraville, a long-established company known as Lamar Walden Automotive has been remedying hot rod troubles since the early 1960s. If convenient, you can reach them at (770) 449-0315.
To read trouble-shooting comments about Blow-through carburetors, read the story above.