Carburetor Diagnosis; Hard Starting; Stalling; Rough Idle - Ford LRG-423 Service Manual

03-22 Auxiliary Systems
03-22
CARBURETOR DIAGNOSIS
General Information
Water and dirt that accumulate in the fuel tank can
cause a restricted fuel line or filter and malfunction of
the fuel pump or carburetor. Condensation, which is
the greatest source of water entering the fuel tank, is
formed by moisture in the air when it strikes the cold
interior walls of the fuel tank.
If the accumulation of dirt and water in the filter is
excessive, the fuel tank should be removed and
flushed, and the line from the fuel pump to the tank
should be blown out. Air leakage in the fuel inlet line
can cause low fuel pump pressure and volume.
A restricted fuel tank vent can cause low fuel pump
pressure and volume and can result in collapsed inlet
hoses or a collapsed fuel tank. High or low pressure
are the two most likely fuel pump troubles that will
affect engine performance. Low pressure will cause a
lean mixture and fuel starvation at high speeds, and
excessive pressure will cause high fuel consumption
and carburetor flooding.
Dirt accumulation in the fuel and air passages,
improper idle adjustments, and improper fuel level are
the major sources of carburetor troubles.
Many problems attributed to the carburetor are caused
by other systems. Proper and careful diagnosis is a
must.
If the problem seems to occur in one engine cylinder,
the ignition system may be at fault. If the problem
seems to occur in all engine cylinders, then the
problem most likely is carburetion. When the
carburetor is thought to be at fault, check the fuel
system first. Dirt or water in the gas tank, leaks in the
fuel lines, or a defective fuel pump often show the
same symptoms as carburetion problems.
Observation, pressure and volume tests on the fuel
system show whether the fuel system is the cause of
the problem.
When the carburetor is shown to be at fault, a simple
cleaning may often solve the problem. Spraying a
carburetor cleaning solvent into the carburetor' s air
intake assembly with the engine running may solve the
problem.
Percolation
Percolation symptoms are a strong smell of fuel
vapors and hard starting of a hot engine that has been
sitting for 20 minutes. Fuel dripping from the end of the
throttle shaft or from the main discharge nozzle
causes a rich mixture in the air intake. Opening the
choke completely and cranking the engine clears the
vapors and eliminates percolation.
Flooding
Flooding occurs from a float level set too high, a
sticking choke plate, float, or float valve, or a fuel inlet
needle held off its seat either by improper seating or
dirt particles. A punctured or leaking float admitting
fuel becomes gas-logged causing a heavy float, high
fuel level and flooding.
If flooding does occur, disassemble the carburetor,
observe the operation and condition of components,
and repair or replace as necessary.
A ruptured accelerator pump diaphragm in th e Zen ith
carburetor will cause a rich mixture condition simila r to
flooding.
Hard Starting
Hard starting when attributed to the carburetor is
caused by a flooded engine due to improper starting
procedures. An improper idle fuel mixture, sometimes
due to improper fuel mixture, or idle fuel mixture
adjusting needles not turned equally in relation to the
other adjusting needle causes hard starting. A too-high
or too-low float level due to misjudgments or wear,
also cause hard starting. Repair, adjust or replace
components as necessary.
Stalling
Improper idle air/fuel mixture or an idle speed set too
low causes an engine to stall. Adjust these systems as
necessary. The stalling problem may be due to a loose
throttle shaft in the bore of the throttle body. Repair as
necessary. Clogged idle air bleeds, idle air passages,
or an idle discharge port cause stalling. Cleaning
eliminates this problem. Carburetor icing is also a
cause of stalling.
Rough Idle
An improper idle air/fuel mixture can cause rough idle.
This may be due to improper adjustment or damaged,
worn or grooved idle mixture adjusting screws. Rough
idle is caused by too high or too low a fuel level due to
floats misadjusted or damaged. A leak between the
carburetor and intake manifold due to a carburetor not
tightened down, a damaged gasket or a crack in either
the carburetor or intake manifold, cause rough idle.
Adjust, repair, or replace as necessary. Clogged idle
air bleeds, restricted idle air passages or idle
discharge ports also cause rough idle. Clean or repair
as necessary.
Surging Above Idle
Surging above idle is caused by a fuel level too low,
due to an improperly adjusted float, clogged main jets,
restricted main jet passages, or main jet having a size
not suited for the application. Adjust, clean, or replace
as necessary.
Poor Acceleration
Poor acceleration is caused by a mixture being too
rich or too lean. If a rich mixture is the cause of poor
acceleration, the fault may be an improperly adjusted
accelerator pump, a fuel level too high due to
damaged or misadjusted floats, fuel inlet needle not
seating due to damage or restriction, restricted air
bleeds, or a worn or damaged main jet. Adjust, repair
or replace as necessary.