Engines produce vacuum, and over the past 130 years engineers have contrived ingenious ways to harness its power to the engine’s induction system. Aided by barometric pressure, ignition and compression, the carburetor creates the air-fuel mixture that promotes combustion. Through a labyrinth of small-bore drillings in the carburetor, the vacuum draws a potent mixture of air and fuel. So formidable is the mixture, the carburetor has empowered naturally aspirated full-bodied 500cu in drag racing cars to speeds in excess of 213mph in a distance no greater than 1,320 feet!
It works by pressure differential; high pressure flows toward an area of low pressure. On starting and at idle, the area of lowest pressure on a carburetor is under the almost closed throttle plates (see second illustration below) and as the engine gains speed, the area of lowest pressure switches to the boost venturii https://www.glass.net (see first illustration).
On 4150-style carburetors, the boost venturii reside within the main venturii and low pressure is created by the constricting shape of their bores, which causes air speed to increase and, as a result, a siphoning effect is introduced at a boost venturi’s tiny air-fuel discharge orifice and through the air-fuel metering circuits back to the fuel bowls. (The faster the air speed passes through the venturii the more fuel is withdrawn from the fuel bowls and thus unleashing optimum engine power.)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.
This pressure difference at the venturii takes effect during the engine’s intake strokes; that is, as the pistons travel downwards on their induction strokes (and with their respective intake valves open), vacuum is created that draws the air-fuel mixture from the carburetor through the intake tracts and into the cylinders.
1. The boost venturii are carefully positioned within the main venturii. The constricted area (that is, the smallest diameter within their bores) serves to increase air speed, which, in turn, lowers air pressure, presenting the perfect placement for the booster’s tiny discharge orifice. Here, the air-fuel mixture is drawn and atomized in the fast-moving air stream to the cylinders.
It is fumes that ignite, as gasoline won’t burn and produce energy in its liquid form. Instead it 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 in the induction tracts. In addition, the carburetor must meet the air-fuel ratio requirements of the engine.
2. The air bleeds draw air into the idle fuel wells and into the main fuel wells where it emulsifies (mixes) before being discharged through the idle ports and transfer slots in the base plate as well as the boost venturii up in the carburettor’s main body.
3. The most common problem that arises in carburetor tuning involves the over exposure of the transfer slots during the idle phase. When increasing the idle speed of a four-barrel modular carburetor, adjust both the primary and the secondary throttle plates. If you adjust the idle speed with the primary throttle blades only, you could upset their position relative to the transfer slots. Some of the adverse effects of an over exposed transfer slot at idle are hesitation, excessive richness, or poor running. In the idle condition, when the throttle plates are closed, the transfer slots should give the appearance of a small square when viewed from underneath.
4. Set the idle mixture to the highest vacuum reading by using a vacuum gauge connected to the constant-vacuum port of the carburetor’s base plate. Slowly adjust the first idle-mixture screw. Make one adjustment only to the first screw. The adjustment should be no more than an eighth or quarter turn. Then leave sufficient time for the carburetor to respond and move to the next adjuster screw. Gradually work your way around the carburetor, making just one, small, slow adjustment to each of the four screws.
In this sequence of pictures and captions, we identify some of the chief components of the modular carburetor, illustrate their functions, acquaint you with common problems that adversely affect them, and tell you how to resolve them. In tuning, always 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 well-established company known as Lamar Walden Automotive has been remedying hot rod troubles since the early 1960s. For those residing in the southeast, you can reach them at (770) 449-0315.
5. The Idle-Eze was created by Demon Carburetion chiefly to ease the complications associated with idle-speed adjustment. This device introduces an extra source of idle air to the engine without disrupting the critical relationship between the idle-speed adjustment screws, the throttle blades, and the transfer slots.
6. After setting the idle mixture check the float levels. With the engine running, the fuel level should be in the middle of the glass window of the fuel bowl. If it isn’t, loosen the lock screw on top of the bowl and adjust the nut clockwise to lower the float level or counter-clockwise to raise it. Make the adjustment by one flat of the nut at a time and wait until the carburetor has had a chance to respond. To check the original dry setting, remove the fuel bowl, turn it upside-down and set the float to this measurement.
7. Installing larger pump shooters often eradicates a hesitation at off-idle, but frequently the fault lies not with the pump shooters at all but with incorrect ignition timing. There are at least two ways of reaching the off-idle position, either gently or suddenly. If the throttle is eased into the off-idle position and the engine stumbles, the idle circuits and the transfer slots are probably too lean. To cure this condition either slightly undo all four idle-mixture adjusting screws to enrich the system or enlarge the idle-feed restrictors in the metering blocks. On the other hand, if the hesitation occurs under rapid acceleration increase the size of the pump shooters. The pump shooters only serve to provide the initial shot of fuel, and together with the idle circuits and transfer slots they provide the predominant fuel supply to the engine until the main circuits are operating through the boost venturii. The pump shooters are also particularly useful during cold starts. One or two depressions of the throttle pedal provide sufficient fuel for starting. A variety of orifices are available, ranging from 0.025in to 0.052in. Usually larger engines use larger orifice pump shooters.
To read trouble-shooting comments about Blow-through carburetors, read the story below: