Introduction

This lecture addresses the funda-

mentals of carburetors, their bene-

fits, limitations and reasons why

they re still a popular bolt-on item.

Some proponents of electronic

fuel injection tend to look upon car-

buretors as dinosaurs. From a

technology perspective, they may

be. But when you consider some of

their cost-effective features and the

range of contemporary refinements

available, today s carburetors are

a far cry from versions introduced

decades ago. It s all about efficient

mixing and proper delivery of com-

bustion-efficient air/fuel blends.

The gap that has traditionally exist-

ed between fuel injection and car-

buretors appears to be narrowing.

Basic Information

THE AIR/FUEL RATIO

REQUIREMENT:

Depending upon an engine s

specific speed and load, optimum

power and efficiency require a cer-

tain mixture of and fuel. That s a

given. The problem is that it diffi-

cult for provide such mixtures

under the myriad of speeds and

loads presented. Contemporary

fuel management systems rely sig-

nificantly on electronics to deliver

amounts of fuel proportional to

carefully monitored inlet airflow.

Fuel injector on time periods are

predetermined by on-board

computer maps that result from

computations that produce certain

best power, best emissions or

best efficiency conditions.

Conventional carburetors do not

enjoy the benefits of pre-deter-

mined fuel patterns or maps.

Instead, they rely upon a pressure

differential created between atmos-

pheric pressure (in the majority of

cases) and cylinder pressure, at

any given engine speed or load

condition. It is this pressure differ-

ential for which a carburetor is

designed to include a combination

of fuel delivery paths or circuits

intended to deliver the proper

air/fuel ratios. Not an easy task, to

say the very least.

These circuits, to be discussed

more in detail a bit later, are

designed to deliver fuel as a func-

tion of airflow rate (pressure drop

across the carburetor). As engine

speed increases and proportional

air passes into the cylinders, addi-

tional fuel is added to the incoming

air. By specific sizing of fuel and

adjoining air passages (sometimes

called air bleeds ), fuel mixed with

air becomes emulsified to allow

improved atomization once it

enters the inlet air stream. The

more efficiently air can be mixed or

atomized, the greater the potential

it will be combusted in the engine s

cylinders. Since liquid fuel will not

burn, combustion efficiency

becomes related to atomization

efficiency, both of which are inex-

orably tied to an efficient carburetor

(originally called a mixing valve ).

Because they are pressure differ-

ential devices, carburetors are sen-

sitive to air flow into or out of an

engine. A classic example of this is

what transpires when an intake valve

first opens. At this time, and for a brief

period, cylinder pressure tends to

be higher than atmospheric pres-

sure. Airflow, therefore, can pass

from the intake manifold, through the

carburetor and back into the atmos-

phere. This back flow or reversion

pressure causes fuel to be delivered

into the air stream, often forming a

cloud of air/fuel mixture just above

the carburetor typically referred to

as standoff and disruptive to opti-

mizing power. This result is often pro-

duced by intake manifolds of too-

small passage volume (as in inde-

pendent-runner or IR manifolds),

exceptionally early intake valve

opening points, excessive exhaust

system backpressure, premature

1 At the Core of Carburetion

At the Core of Carburetion

“

The more efficiently

air can be mixed or

atomized, the greater

the potential it will

be combusted in the

engine’s cylinders.”

Jim McFarland

Join Jim McFarland as he steps into his role

as the "Performance Professor" and shares

a wealth of knowledge and experience that

will help take you to a higher level of per-

formance.