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  The faster the air speed in the venturii, the more fuel is withdrawn from the bowls, thus unleashing optimum engine power.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 are open), vacuum is created in a progression of gulps, drawing 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 (partial vacuum), 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.

Power Valve

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:

Here 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.

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.

5. The Idle-Eze was created by Demon Carburetion to ease the complications associated with idle-speed adjustment. This device can introduce an extra source of idle air to the engine without disrupting the critical relationship between the idle-speed screws, the throttle blades, and the transfer slots.

6. Float levels are often too high; they should be placed in the middle of the fuel bowl sight glass. The initial float setting when the carburetor is being assembled at the Demon factory (the dry setting) is accomplished by removing the fuel bowl and, with it turned upside-down, the dimension between the inside top of the bowl and the top of the float is set at approximately 0.375in to 0.0400in

7. Installing larger accelerator pump discharge nozzles often eradicates a hesitation at off-idle, but frequently the fault lies not with the pump discharge nozzles 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 discharge nozzles. These nozzles 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 discharge nozzles 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 discharge nozzles.

8. A vacuum-operated fuel enriching device, the power valve (illustrated above and below) is located in the carburetor’s metering block between and above the main jets. It’s immersed in the fuel bowl on one side and occupies a source of vacuum on the other. Power valves are rated at half the intake manifold vacuum when observed at idle. For example, an intake manifold vacuum reading of 13in calls for a power valve rated at 6.5in.  The 6.5 power valve is identified with the numerals 65 stamped upon it (barely visible in the lower left image).  With an idling engine at normal operating temperature, the vacuum gauge can acquire a reading from vehicles with manual transmissions while in neutral. Automatic transmissions require the engine idling in gear with an assistant’s foot on the brake pedal.

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.

Blow-through carburetors: 650, 750, & 850cfm Mighty Demons

To read trouble-shooting comments about Blow-through carburetors, read the story above.