Carburetion Troubleshooting Detail Reference Guide

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Carburetion Troubleshooting
                               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
                            from running.
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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