|
Big, small, simple or complex, all carburetors share the same basic operating
principles. If you make the effort to get a firm grasp of those principles,
you'll be able to diagnose any specimen you happen to encounter even if it's
unfamiliar.
Essentials
Knowledge of some simple physics is a necessary foundation. To begin with,
only gasoline vapor will burn, so fuel must change its state from a liquid to a
gas at some point or the engine won't run, and to do this it must absorb enough
heat to boil. You may wonder how gasoline can boil when both the engine and the
air are cold. The answer is reduced pressure. Just as water boils at a lower
temperature on a mountain top than it does at sea level because there's less
atmospheric pressure bearing down on it, so the vacuum in the carburetor's
venturi and intake manifold causes fuel vaporization to occur when very little
heat is present. The boiling point is reduced so that the fine atomized droplets
that are sprayed into the intake stream vaporize from the latent heat in the
air, no matter how little that might be. Of course, until the engine has warmed
itself up, only a small portion of the available gasoline actually turns to
vapor, which makes a very rich mixture as provided by the choke
necessary.
Ratio
Air/fuel ratio is expressed in terms of weight, so a 15:1 ratio means 15
pounds of air to one pound of gasoline (by volume, that would be about 2,000
gallons of air to one gallon of fuel). The ideal "stoichiometric"
ratio in which exactly the right amount of air is present to burn the fuel is
actually 14.7:1, but maintaining such perfection requires the use of an oxygen
or lambda (the Greek letter that has come to represent the ideal blend) sensor
and electronically-controlled feedback or closed loop mixture adjustment, a
subject covered in other sections of this encyclopedia.
Any ratio from about 8:1 to 18:1 will fire dependably. In the former,
there'll be more gasoline present than is needed, so all the air will be used up
while much of the fuel will find nothing to combine with (it will be pumped raw
into the exhaust system). In the latter lean
mixture, there's more air present than is necessary to burn the fuel, so all the
gasoline will be used up. It's important to realize that the actual reaction (the
rapid oxidation of the fuel) always occurs at the 14.7:1 ratio regardless of the
mixture that's actually supplied by the carburetor.
Venturi action
Whenever air passes through a tube, a pressure drop occurs, and this is the
principle that moves fuel into the throat of a carburetor. But since the
strength of the vacuum is directly proportional to the speed of the air column,
some kind of boost is needed at low rpm, and that's the reason for the venturi.
By placing a restriction in the throat, the air is forced to move faster and an
extra pressure drop is created allowing atmospheric pressure on the fuel in the
bowl to push enough through the nozzle to permit the engine to run. Vaporization
is also enhanced.
The above should help you understand the following explanations of the six
systems found in almost every carburetor.
Reservoir
All the gasoline an engine uses makes an intermediate stop in the bowl, which
supplies the idle, cruising, and power circuits, and the accelerator pump. A
needle-and-seat valve and float arrangement keeps the bowl from over-filling
when the engine is not using all the fuel the pump supplies. It may not seem
that a bit of plastic or a tiny metal pontoon would have the buoyancy necessary
to shut down the pump's considerable pressure, but it's affixed to a lever in
such a way that it has plenty of mechanical advantage and can push the needle
into its seat hard enough to do the job easily.
At idle
Since there's very little air passing through the venturi at idle, not enough
vacuum is generated to move the fuel through the cruising system, so the idle
circuit has to take over. This comprises a port below the throttle plate where
there's plenty of vacuum, a passage from the bowl, and an adjustment screw. The
velocity of the incoming air is low with the throttle almost closed, so the
passage to the port usually has air bleed or emulsion holes in it to aid in
atomizing the gasoline (it would be difficult to get a solid stream to vaporize).
Some carburetors have idle air jets so that the throttle plates can close
completely, somewhat similar to the idle air bypass of a typical fuel
injection system.
Hot air is relatively thin, so when the temperature is very high not enough
air molecules are getting around the throttle plates at idle to make the proper
blend. To eliminate the excessive richness this would cause, some carburetors
have a hot idle compensator, which is simply an air passage that bypasses the
throttle plates whenever a temperature-sensitive bimetal valve opens.
Other fuel passages are needed to provide a smooth transition from idle to
moderate rpm, and these are called transfer or off-idle ports or slots. They are
positioned higher up in the barrel than the idle port and are progressively
uncovered and exposed to vacuum as the throttle plate opens. They generally get
fuel from the same tube as the idle port, but are not affected by the mixture
screw.
Cruise and stomp
The next circuit to come into action is the main cruising system. This is a
nozzle that sprays gasoline into the part of the venturi or venturis where the
highest vacuum is present. It gets its supply of fuel in an amount controlled by
the diameter of the main jet. This circuit works constantly at steady speeds and
is calibrated for good gasoline mileage.
Whenever the driver asks the engine to provide all the power possible by
pushing the accelerator pedal to the floor, thus opening the throttle plate all
the way, too much air enters the engine for the cruising circuit to handle. The
mixture would lean out and output would be severely limited. So, the aptly-named
power circuit is activated. This can be either a separate fuel valve
or a metering or step-up rod that normally blocks some of the main jet's flow,
but is pulled up out of the way by linkage or a spring
when extra fuel is needed (the rod may be held down against a spring by a vacuum
piston, then rise when the vacuum drop that occurs at wide open throttle reduces
the force on the piston).
Lag eliminator
The circuits mentioned so far would be sufficient if the engine were only
asked to run at a constant speed or to accelerate very gradually. That, however,
is not the way cars are driven. The throttle is often opened too rapidly for the
above systems to be able to keep up. The sudden blast of air would cause the
engine to stumble or stall before enough fuel could be moved into the intake
stream to provide a burnable mixture.
The accelerator pump is what adapts the carburetor to the realities of the
highway. It squirts an extra charge of gasoline into the intake stream whenever
the throttle is opened, and, since it works mechanically, does so before the
engine gets a chance to choke on too much plain air. It's an ordinary pump with
one-way inlet and outlet valves and an air bleed, weight, or spring set-up to
eliminate the possibility of fuel escaping from its nozzle because of vacuum.
Super rich
Finally, there's the choke, a device that gets the engine started even when
it's too cold for the proper amount of gasoline to vaporize. By closing off the
mouth of the carburetor so the manifold vacuum present during cranking causes a
great deal of fuel to flow out of the bowl into the throat, enough vapor is
available to allow the engine to fire. Once running, the power plant starts
producing sufficient vacuum to act on the diaphragm or piston of the choke
pull-off mechanism, opening the choke enough to permit an adequate amount of air
to enter the manifold for fast idle and cruising operation. As the engine warms
up, a calibrated coil of flat metal that's connected to the choke plate expands
from the heat of exhaust or coolant
or an electrical element. Gradually, this expansion opens the choke until air
flow is no longer restricted. |
