by the choke unloader (fig. 4-38). The choke unloader
can be either mechanical- or vacuum-operated.
A mechanical choke unloader physically opens the
choke plate any time the throttle swings fully open. It
uses a metal lug on the throttle lever. When the throttle
lever moves to the fully opened position, the lug pushes
on the choke linkage (fast idle linkage). This provides
the operator a means of opening the choke. Air can then
enter the air horn to help clear a flooded engine (engine
with too much liquid fuel in the cylinders and intake
A vacuum choke unloader (fig. 4-39). also called a
choke brake, uses engine vacuum to crack open the
choke plate as soon as the engine starts. It automatically
prevents the engine from flooding.
Before the engine starts, the choke spring holds the
choke plate almost completely closed. This action
primes the engine with enough fuel for starting. Then as
the engine starts, the intake manifold vacuum acts on
the choke brake diaphragm. The diaphragm pulls the
choke linkage and lever to swing the choke plate open
slightly. This action helps avoid an overly rich mixture
and improves cold engine drivability.
Figure 4-38.Choke unloader.
Figure 4-39.Vacuum choke unloader.
There are 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
Fast Idle Solenoid
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.
When the 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.
Throttle Return Dashpot
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