Detail Reference Guide
Air Cleaner Manifold Problems involving the air cleaner manifold are normally related to loose mounting screws
and/or damaged or leaking gaskets. A leaking gasket allows unmetered air to enter the
intake system. Symptoms, based on severity of the leak, can range from an overly lean
condition where the engine will have trouble accepting or handling load to the engine won't
start or run at all. See Backing Plate Gasket
Air Cleaner, Restricted Restricted air filter elements effect engine operation differently based on the carburetor
venting system. The mixture for Internally vented carburetors typically becomes lean while
the fuel mix for externally vented carburetors becomes overly rich. Based on the carburetor
design, effects can range from an engine that loads up and runs overly rich to an engine that
can't handle a load because there is not enough fuel available.
Air Intake, Restricted See A/C, restricted
Automatic Choke The automatic choke function of the Pulsa-Jet carburetor used on vertical crankshaft
lawnmower style engines can cause an overly rich condition during starting and operation.
The default choke position is on. If the diaphragm stiffens over time, it will not allow the
choke plate to open smoothly and the choke can stay on longer than it should. If the
diaphragm has a hole at the link or through the material, the choke will not disengage as it
should. A small vacuum metering jet into the diaphragm spring cavity can become plugged,
defeating the choke pull off.
Backing Plate Gasket A leak in an air cleaner backing gasket means, among other things, that unmetered air is
entering the engine and leaning out the fuel mixture. This can result in power and
performance issues. Some primer equipped carburetors use the air cleaner backing plate
gasket to seal a passage through which air pressurized by the primer bulb travels. If this
pressurized air escapes, the priming pulse is lost and the engine may be hard to start or will
not start. See Air Cleaner Manifold.
Bi-Metal Choke Spring Used on vertical crankshaft lawnmower type engines with Pulsa-Jet and Vacu-jet
carburetors, the Bi-metal spring aids the choke in staying closed to augment cold starts and
assists the choke in opening when the engine is warm.
Bowl Gasket Seals the carburetor bowl to the carburetor body. An improperly seated or damaged bowl
gasket can result in fuel leaks.
Bowl Nut On one-piece Flow-Jet carburetors, the bowl nut secures the float bowl to the carburetor
body and seals against leakage via the bowl nut gasket. Should damage to the gasket
occur, external fuel leakage is probable.
Bowl Vent Air at normal ambient pressure is allowed access to the float bowl via the bowl vent. The
vent assures smooth fuel delivery. As fuel is removed from the bowl by the carburetor
circuits, volume is being removed. Unless air can enter to take up the space, the pressure in
the bowl will drop, effecting fuel delivery through the circuits. An engine with an obstructed
bowl vent will act as though it is running out of gas. Engine symptoms can be hard starting,
hesitation, and hunting and surging.
Check Valve Pulsa-prime carburetors. This wet bulb primer incorporates a check valve in the priming
circuit that allows fuel to fill the bulb via the fuel supply but closes when the bulb is
depressed. If the valve leaks or does not work, there will be reduced or no prime and a hard
or no start condition.
Choke Control See Choke Linkage, Choke Control Cable
Choke Control Cable Both the throw of the choke control cable and the location of the cable clamp will effect the
operation of the choke mechanism. If the choke will not fully close, cold starting can be
difficult. If it does not fully disengage, engine operation can be effected.
Choke Linkage Choke linkage that is misadjusted or damaged can cause improper engagement or
disengagement of the choke mechanism. If the choke will not fully close, cold starting can
be difficult. If it does not fully disengage, engine operation can be effected.
Choke Shaft One of the few moving parts in the carburetor, the choke shaft determines the range of
motion of the choke plate. Should the shaft become restricted in its travel, the choke plate
will correspondingly be limited in motion. Without full choke engagement, an engine may be
hard to start or not start at all.
Contaminated A carburetor should contain nothing but fuel and air. The presence of anything else
suggests that the carburetor is contaminated. In terms of fuel contamination, this can be
anything in the fuel that does not belong there such as dirt, debris, varnish, water, etc.
Debris in Carburetor Debris, also grouped with dirt, can be considered as any loose material found in fuel systems
that is not fuel. Some dirt and/or debris will cause no problems because it is small enough to
pass through all the orifices. Of concern is material that blocks or limits fuel or airflow.
Effects will vary based on the degree of blockage. Minor blockage of a fuel passageway
may result in a slight hesitation. Complete blockage can result in a no start condition.
Debris in air passageways may result in hesitation or overly rich running conditions.
Float Found in Flow-Jet carburetors, the float controls the operation of the inlet needle and seat.
As the name implies, the float "floats" on top of the fuel in the carburetor bowl. Based on the
float level adjustment, The float position will either shut off the fuel supply via the inlet needle
and seat or pull the
Float Adjustment See float level. Most floats are non adjustable. If adjustment is possible, it typically involves
bending a tang on the float bracket that will alter the float position.
Float Level See Float. The float level establishes a base line for carburetor calibration/adjustment. The
low pressure signal in the carburetor venturi is the force that accomplishes the work of
moving fuel from the bowl to the carburetor throat. The lower the fuel level, the higher it must
be lifted. To move the same amount of fuel, the pressure must drop even lower. Engine
operating symptoms of a carburetor with a float level too low can be hesitation and inability
to handle load due to a mixture that is too lean. A float level set too high will usually leak
fuel. If the engine does operate, it may show signs of a mixture set to rich such as the
engine loads up, exhaust gas smells rich, spark plugs foul easily, etc.
Fuel Delivery Volume, Low If fuel delivery is too low, the engine will typically display signs of running too lean.
Fuel Filter, Leaks Fuel leaks can occur at fuel filter connections or through a cracked or damaged filter body.
Fuel Filter, Restricted Fuel filter restriction can result in varied symptoms based on the severity of the restriction.
Minor restrictions can cause hunting and surging. Major restrictions will prevent the engine
Fuel Level Low Fuel flow occurs in gravity feed fuel systems because the fuel level is higher in the supply
tank than it is in the carburetor bowl. The pressure or force that moves fuel from one area to
the other is called head pressure. The closer the levels become, the lower the pressure.
Home made or modified systems may not have enough head pressure to move enough fuel
to the carburetor when the tank level becomes low. See Float Level, Fuel Tank Mounting
Fuel Line Issues involving the fuel line will usually be fuel supply restrictions or fuel leakage. Fuel
restrictions can be the result of a kink or sharp bend in the line, which will either stop or slow
fuel delivery or a high point in the line, which can be the cause of vapor lock. Poor
connections, loose clamps or old, stiff or cracked fuel line can be the cause of external fuel
Fuel Pump When used, the fuel pump is the primary fuel delivery device. Pump problems will obviously
result in fuel supply issues. These may range from intermittent, all of the time or just under
certain conditions such as high, outside ambient temperatures.
Fuel Quality Old, stale gasoline may not even burn. Fuel quality can be responsible for no start, hard
start or power and performance conditions.
Fuel Shut-Off Valve A fuel shutoff valve must be all the way open to allow full fuel delivery. Anything less may be
responsible for a restriction in fuel delivery. See Fuel Line, Fuel Delivery Volume Low.
Fuel Solenoid Some systems will have an inline fuel solenoid, which will stop fuel delivery to the carburetor
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