several devices used on carburetors to improve drivability and economy. These devices are
as follows: the fast idle solenoid, the throttle return dashpot, the hot idle compensator,
and the altitude compensator. Their applications vary from vehicle to vehicle.
A fast idle solenoid, also known as an antidieseling solenoid (fig. 4-40), opens the
carburetor throttle plates during engine operation but allows the throttle plates to close
as soon as the engine is turned off. In this way, a faster idle speed can be used while
still avoiding dieseling (engine keeps running even though the ignition key is turned
off). This is a particular problem with newer emission controlled vehicles due to higher
operating temperatures, higher idle speeds, leaner fuel mixtures, and lower octane fuel.
engine is running, current flows to the fast idle solenoid, causing the plunger to move
outward. The throttle plates are held open to increase engine speed. The plunger is
adjustable, so the idle speed can be adjusted. When the engine is turned off, current flow
to the solenoid stops. The solenoid plunger retracts and the throttle plates are free to
swing almost closed.
The throttle return dashpot, also known as an antistall dashpot (fig. 4-41), acts as a
damper to keep the throttle from closing too quickly when the accelerator pedal is
suddenly released. It is commonly used on carburetors for automatic transmission equipped
vehicles. Without the throttle return dashpot, the engine could stall when the engine
quickly returned to idle. The drag of the automatic transmission could kill the engine.
return dashpot works something like a shock absorber. It uses a spring-loaded diaphragm
mounted in a sealed housing. A small hole is drilledinto the diaphragm housing to prevent
rapid movement of the dashpot plunger and diaphragm. Air must bleed out of the hole
vehicle is traveling down the road (throttle plates open), the spring pushes the dashpot
plunger forward. When the engine returns to idle, the throttle lever strikes the extended
dashpot plunger, and air leaks out of the throttle return dashpot, returning the engine
slowly to curb idle. This action gives the automatic transmission enough time to
disconnect (torque converter releases) from the engine without the engine stalling.
A hot idle compensator (fig. 4-42) is a
thermostatically controlled device that prevents engine stalling or a rough idle under
high engine temperatures. The temperature sensitive valve admits extra air into the engine
to increase idle speed and smoothness.
engine temperatures, the hot-idle compensator valve remains closed, and the engine idles
normally. When temperatures are high (prolonged idling periods, for example), fuel vapors
can enter the air horn and enrich the air-fuel mixture. The hot idle compensator opens to
allow extra air to enter the intake manifold. This action compensates for the extra fuel
vapors and corrects the air-fuel mixture.
An altitude compensator is used to change the air-fuel mixture in the carburetor with
changes in the vehicle height above sea level. Normally the compensator is an aneroid
device (bellows device that expands and contracts with changes in atmospheric pressure).
As a vehicle
is driven up a mountain, the density of the air decreases. This condition tends to make
the air-fuel mixture richer. The reduced air pressure causes the aneroid to expand,
opening an air valve. Extra air flows into the air horn and the air-fuel mixture becomes
leaner. The opposite occurs when the vehicle descends from the mountain. The greater air
density and pressure tends to make the carburetor mixture too lean. The increased air
pressure collapses the aneroid and the air valve closes. This action enriches the mixture
enough to compensate for the low altitude.