The Four-Barrel Chronicles

June 1st, 2011

Text and Photos by Dave Emanuel

More Carburetor Mysteries Solved, Part II

Appearance Vs. Reality

Changing priorities during the 1970s and ‘80s combined with the EPA’s grand and glorious entrance into the automobile business, made fuel distribution especially important.  It is for this reason that some carburetors, which were used both as original equipment and for aftermarket replacement, differ even though they share the same model designation. New car emissions standards may have dictated internal changes not required on performance-type replacement carburetors produced during the same time period.

Since each individual booster design creates a vacuum signal of different intensity, it can be seen that fuel metering requirements and air bleed dimensions can vary significantly between carburetors, even among those with identical throttle bore and venturi diameters.  And by dealing with these variables, carburetor engineers can create an almost infinite variety of fuel delivery curves. If you plan to purchase a used carburetor of unknown origin, keep the possible design variations in mind and do a little research; otherwise, you might end up with a carburetor that’s inappropriate for the engine on which you plan to install it.

Jets—The Keys to Mixture Control

Without the proper equipment, altering the air bleeds or booster venturis is tantamount to playing with fire (which can be quite dangerous around carburetors; I’ve had the singed eyebrows that prove it). Except for some specifically designed race carburetors, these items are simply not designed to be modified, and misdirected enthusiasm can ruin an otherwise functional carburetor. However, the main metering jets can be removed and replaced by anyone who understands the operation of a screwdriver. Changing fuel-metering jets is therefore the best means of altering fuel flow to suit a specific requirement.

The carburetor’s primary jets are precisely machined to provide a specific amount of fuel flow. In addition to orifice diameter, entry and exit contour also influence flow capacity. Consequently, drilling a jet opening to a larger diameter doesn’t always provide the desired result.

A carburetor jet is nothing more than a metering device that controls the flow of fuel from the fuel bowl into the main circuit. After passing through a jet’s orifice, fuel travels into the main well, where it is emulsified (mixed with air from the air bleeds). It then moves on to a connecting channel that leads to the discharge nozzle.

Power Enrichment Circuit

So long as the fuel-metering signal remains constant or increases in intensity (for a corresponding increase in vacuum) fuel flow will be sufficient to maintain the air/fuel ratio as determined by the metering jet/air bleed/venturi relationship. But what happens when signal strength drops, as often happens when the throttle plates are opened quickly?  Obviously, a leaner air/fuel mixture will result: As manifold vacuum decreases, the amount of fuel drawn through a fixed orifice (the jet) also decreases. Consequently the mixture becomes lean at precisely the time when acceleration demands a richer mixture.

Engine vacuum also drops during periods of extended high load —for example, when climbing a hill or towing a trailer—so a carburetor must be designed to meter additional fuel for all heavy-load conditions, irrespective of how long that load may be applied.  Depending on your point of view, additional fuel for high-load conditions is added by either a power enrichment or economizer circuit.

Carter, Edelbrock and Rochester carbs use metering rods and spring-loaded pistons to control power enrichment fuel flow. Holley carburetors rely on a power valve for on/off enrichment fuel control and channel restrictions for metering flow.

Economizer or Powerizer?

In days of old, and occasionally in days of new, carburetors were said to have an “economizer circuit.” This nomenclature is somewhat backwards since it is an apt description of the circuit only when it is not enabled. The circuit is actually designed to enrich the fuel mixture when needed, not to further lean the mixture, as might be suggested by a label containing a reference to economy. “Economizer” is undoubtedly the product of a brilliant marketing mind whose owner was attempting to justify the added expense of another fuel circuit to a parsimonious, all-conquering accounting department. (Being given more to extravagance than frugality, I’ll take the positive approach and emphasize power.)

As used in carburetors equipped with metering rods (most commonly those produced by Carter/Edelbrock and Rochester), the power enrichment circuit consists of a spring-loaded piston to which is attached a tapered metering rod (also called a step-up rod). The metering rod protrudes through the main jet orifice, thereby limiting fuel flow. During periods when engine vacuum is high, as during highway cruise, the piston is pulled to its full downward position against the return spring. This movement allows the larger diameter section of the metering rod to enter the main jet, thereby limiting fuel flow (as opposed to an unrestricted jet) and establishing the desired air/fuel ratio for light load conditions. Conversely, as vacuum drops below the point where spring pressure is overcome, the piston moves upward, pulling the large diameter part of the rod out of the jet, allowing additional fuel flow through the main jet.

Holley carbs typically use an externally mounted, diaphragm-style accelerator pump that allows discharge rate and volume to be easily adjusted.

Spare the Rods

Instead of metering rods, Holley and Autolite/Motorcraft carburetors typically incorporate a power valve circuit to administer additional fuel for high load engine operation. When manifold vacuum exceeds the valve’s rated opening point, it draws the valve to the closed position. Conversely, when manifold vacuum drops below a predetermined level, spring pressure pops the valve open.

Implementation of a power valve actuated enrichment circuit is quite straightforward.  When vacuum drops to a sufficient level, spring pressure working against a diaphragm opens the power valve and allows additional fuel through orifices, generally known as PVCRs (power valve channel restrictions). These restrictions meter the flow of fuel into the main well and consequently control the amount of fuel enrichment admitted through the main circuit.

A power valve circuit can suffer from a rupture of the rubber diaphragm (not uncommon), and leaks between adjacent passages. However, a metering rod design is virtually impervious to the effects of time, wear and engine backfire. Failures in metering rod enrichment circuits are rare, and are usually the result of the piston becoming seized in its bore, or blockage of the passage that allows manifold vacuum to control piston position, often from a long overdue rebuild. In both cases, malfunctions are normally corrected by removing excessive carbon and varnish buildup.

Accelerator Pump Circuit

If the same creative soul who contrived the economizer valve label had been allowed to run rampant, he would have no doubt blessed the accelerator pump with an equally obtuse name. Fortunately, after the economizer episode, he was trundled off somewhere far removed from carburetor nomenclature and spent his remaining years in a small, bleak cubicle attempting to balance long columns of debits and credits. As a result, an accelerator pump is an appropriately named item designed specifically to pump fuel into the engine during acceleration. And this remarkable feat of linguistic logic is universally held, except in Great Britain where English is considered a foreign language.

An accelerator pump serves to provide a quick shot of fuel anytime the throttle is opened abruptly. The volume of fuel discharged through the pump nozzle or squirter into the air stream is determined by the degree and quickness of throttle shaft opening. The greater the degree of opening and the faster the opening rate, the more fuel will be discharged. If the throttle is opened slowly, fuel is allowed to return to the float bowl rather than being pumped out of the discharge nozzle. And even though this function could be termed a “controlled leak” like mechanical fuel injection, it functions precisely as intended.

When a standard or mild performance engine is idling, manifold vacuum will generally be between 14 and 17 in-Hg. This high level of vacuum will maintain a satisfactory fuel flow through the idle circuit. If the throttle plates are opened quickly (as in normal acceleration) several events, which serve to disrupt the steady mixing of air and fuel, occur concurrently:

A)  Manifold vacuum drops near or to zero, so fuel flow through the main discharge nozzle virtually stops

B)   Fuel already in suspension returns to a more liquid state and falls out of the air stream

C)  The new throttle setting requires a significant increase in fuel flow, so the main system and/or power enrichment circuit must be activated.  However, without a reasonably strong manifold-vacuum signal, flow will not be initiated.

Bridging the Gap

Without an accelerator pump to supply additional fuel during throttle opening, momentary fuel starvation would cause the engine to sputter, backfire or die. In essence, this circuit fills a gap in the fuel delivery capability of carburetors used in almost all automotive applications. It should be noted, however, that some low airflow capacity carburetors—designed for small-displacement engines—do not have accelerator pumps. In these special applications, large booster venturis, combined with small throttle bore diameters, are capable of maintaining sufficient vacuum to promptly activate the main system and prevent a momentary lean-out. But almost all carburetors designed for performance or racing— because of their large throttle bore diameters—use accelerator pump circuits.

During the transition from idle circuit to main system metering, fuel discharged through the accelerator pump compensates for the time lag that exists between demand (throttle opening) and supply (main system start-up). This lag is present only when the throttle is opened quickly.  Alteration of the pump discharge nozzle diameter, pump capacity or pump actuation lever geometry influences both the volume and timing of the squirt (how much, how early/late and how long).

Engine Load and the Pump Circuit

One important factor that is not often considered in the tailoring of the accelerator pump circuit is engine load. Many times an engine will accelerate smoothly when the transmission is in neutral, but will stumble badly when called upon to actually move a vehicle. Absence of a load enables engine rpm, and airflow to increase quickly when the throttle is opened. Therefore, manifold vacuum doesn’t drop very much, so the fuel signal remains strong. This combination of high airflow, relatively strong vacuum signal and a rapid increase in rpm initiates main system fuel flow quickly. Conversely, when a load is present, airflow and vacuum remain low for a considerably greater time period, the vacuum signal drops in intensity, and fuel flow isn’t sufficient to maintain smooth engine operation.

The duration of the low vacuum signal time during vehicle acceleration is influenced by many factors: engine torque, vehicle gearing, vehicle weight, ignition timing, cam timing, carburetor size and the engine rpm range. For these reasons, tuning the accelerator pump circuit is often necessary for optimum engine operation, for as low-signal (or lag) time increases, the volume of fuel discharged at the pump nozzle and the duration of the pump shot should also be increased. In essence, the fuel delivered by the accelerator pump must keep the engine supplied with the correct air/fuel ratio mix during throttle transition times, when the fuel available from other carburetor circuits is inadequate.

Idle System

When applied to a piece of machinery, or the chronically unemployed (or sometimes to individuals who are employed), the term “idle” refers to a rest or non-operating condition.  However, when an internal combustion engine idles, it is not at rest, but merely running at the lowest speed in its usable operating range. Some race engines idle at rpm levels that are in excess of the cruise speeds of stock engines, so idle is a relative term, and “acceptable idle speed” is largely dependent upon the application for which an individual engine is built and tuned.

The engines in most passenger vehicles are expected to idle smoothly at relatively low rpm. Performance enthusiasts will typically accept (and usually relish) some amount of roughness at idle as a trade-off for increased high-rpm horsepower. Race car owners will accept all of the idle roughness they can get, if it’s part of the trade-off for more power at the upper end of the rpm scale. With these parameters, it is easy to understand why idle circuit calibrations differ drastically between carburetors. It should also be obvious that modifications to the idle system will almost always be required when a carburetor is used in an application for which it was not originally intended.

Dilution of the intake charge by exhaust gases that have not been purged from the combustion chamber is one of the phenomena of the internal combustion engine, and it is exacerbated by high-performance camshafts. This condition is most severe at low engine speeds, but dissipates as engine rpm approaches the point of optimum operating efficiency.  Low-speed dilution necessitates that idle mixtures be richer than cruise air/fuel ratios in order to maintain some degree of smoothness. Typically, as camshaft duration and overlap are increased (i.e., the cam profile becomes more radical) idle fuel jetting (idle feed restriction size) must be correspondingly increased to compensate for mixture dilution and over-scavenging (an additional ill-effect of high-performance cam profiles operated at low speeds, resulting in an amount of the unburned intake charge being drawn out through a late-closing exhaust valve). It is for this reason that carburetors designed for race and high-performance engines frequently contain an idle fuel calibration that is far too rich for a street engine.

The next issue of Drag Racer will contain the final chapter of “The Four-Barrel Chronicles” and will cover idle circuit, chokes and cold starting, and secondary throttle operation.

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