© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois by Russell Krick

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Basic carburetor  Carburetor problem diagnosis

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Bolts to the intake manifold  Mixes air and fuel in correct proportions  When the engine is running, downward- moving pistons on the intake stroke produce vacuum in the intake manifold  Air rushes through the carburetor into the engine  The carburetor meters fuel and mixes it with the air

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Basic Carburetor Parts

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Body  Cast metal housing that contains cast and drilled passages for air and fuel  Contains the main discharge tube, fuel bowl, and venturi

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Air Horn  Also called the throat or barrel  Routes outside air into the intake manifold  Contains the throttle valve, venturi, and discharge tube

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Throttle Valve  Butterfly valve in the air horn  Controls airflow through the carburetor  Controls power output

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Venturi  Produces suction to pull fuel out of the main discharge tube  Narrowed airway increases air velocity, forming a low pressure area  Vacuum is highest inside the venturi

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Basic Carburetor Systems  Float  Idle  Off-idle  Acceleration  High-speed  Full-power  Choke

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Float System  Maintains the correct fuel level in the fuel bowl  Components:  float  fuel inlet valve (needle and seat)  bowl vent

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Float System Float controls the needle valve

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Float System Operation  The float rides on the fuel  When the fuel level reaches the specified level, the float forces the fuel inlet valve (needle and seat) closed  When the fuel level drops below the specified level, the float rides lower, allowing the needle valve to drop away from its seat  the fuel pump can then fill the bowl

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System  Provides mixture at speeds below approximately 800 rpm or 20 mph (32 km/h)  Components:  low-speed jet  idle air bleed  idle passage  idle mixture screw

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System High vacuum pulls atomized fuel out of the idle port

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System Operation  Throttle valve is almost closed  Airflow is too restricted to produce vacuum in the venturi  Idle circuit uses manifold vacuum to feed fuel below the throttle valve  Low-speed jet limits fuel flow in the idle circuit

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System Operation  Idle air bleed adds air bubbles to fuel  helps break up, or atomize, fuel  makes the air/fuel mixture burn more efficiently  Idle passage carries atomized fuel to the idle screw port  the idle screw port is the opening in the air horn below the throttle valve

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle Mixture Screw  Allows adjustment of the idle port  turning the screw in reduces the size of the idle port opening, leaning the mixture  turning the screw out enlarges the size of the idle port opening, enriching the mixture

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Off-Idle System  Feeds more fuel into the air horn when throttle plates are partially open  Extension of the idle system  Functions above 800 rpm or 20 mph (32 km/h)  Supplies fuel during transition from idle circuit operation to high-speed circuit

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Off-Idle System Operation As the throttle is opened slightly, the system feeds extra fuel to the increased airflow

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Acceleration System  Provides extra fuel when changing from idle circuit to high-speed circuit (main discharge) operation  Operates when the accelerator is pressed

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Accelerator Pump  Develops pressure to force fuel out of the pump nozzle into the air horn  Two types:  piston  diaphragm

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Acceleration System Squirts fuel into the air horn every time the throttle is opened

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only High-Speed System  Also called the main metering system  Supplies the engine’s air-fuel mixture at cruising speeds  Functions from about mph (32-90 km/h) or rpm  When airflow is high, venturi vacuum pulls fuel through the high-speed jet, and out of the main discharge tube

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only High-Speed System Venturi vacuum pulls fuel through the main discharge

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only High-Speed Jet  Calibrated size hole in a removable insert  Hole size determines how much fuel flows through the circuit

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Full-Power System  Enriches mixture for high-speed, high- power conditions  Operates when the accelerator pedal is pushed down for full power  Uses either a metering rod or a power valve to provide variable high-speed air-fuel ratio

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Metering Rod  Provides high-speed fuel mixture  Moves in and out of the main jet to vary fuel flow

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Full-Power System When the throttle is opened, the metering rod is lifted out of the main jet

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Power Valve  Serves the same function as the metering rod  Serves as an extra jet, feeding fuel into the high-speed circuit  Components:  fuel valve  vacuum diaphragm  spring

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke System  Supplies rich air-fuel mixture to aid cold engine starting and running  Components:  choke plate (butterfly valve)  thermostatic spring

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Basic Choke System

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Cold Engine Operation  When the engine is cold, the spring holds the choke closed  At startup, high vacuum below the choke pulls a large amount of fuel out of the main discharge

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Hot Engine Operation  When the engine warms, heat uncoils the spring and opens the choke  this produces a leaner mixture  a warm engine will not run properly if the choke remains closed

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke Heat Sources  Electric assist  hot air and electric heating element  All electric  two stage heating element

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Fast idle cam  Fast idle solenoid  Throttle return dashpot Carburetor Devices

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Fast Idle Cam  Fastened to the choke linkage  Increases idle speed when choke is closed

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Fast Idle Solenoid  Holds throttle open when the engine is running  Allows throttle to close when turned off  Avoids run-on, or dieseling

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Throttle Return Dashpot  Causes throttle plate to close slowly, preventing stalling  Often called an anti-stall dashpot

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Barrels  Primary  operate under normal driving conditions  Secondary  function under high engine power output  opened by secondary diaphragm

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Barrels More barrels are used on larger engines

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Size  Stated in cubic feet of air per minute (CFM)  Determines the amount of air that can flow at wide-open throttle

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  The computer calculates air-fuel mixture based on signals from various sensors  Electrical signals activate the mixture- control solenoid Computer-Controlled Carburetors

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Mixture Control Solenoid Quickly opens and closes a valve to change air-fuel ratio

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Visually inspect carburetor  Check for leaking fuel, sticking choke, binding linkage, and missing or disconnected vacuum hoses  Check the rest of the engine compartment for disconnected wires and vacuum leaks

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Inspection External check may find the source of the problem

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Problems  When diagnosing carburetor problems, try to determine which system is at fault  Use a logical procedure to narrow down the possible sources of problems

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Symptoms:  flooding  rich mixture  lean mixture  fuel starvation  stalling  engine miss Float System Problems

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only High Float Level  Symptoms:  rich mixture  high fuel consumption  black exhaust smoke Low Float Level  Symptoms:  lean mixture  high speed miss  stalling when cornering

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System Problems  Symptoms:  rich mixture  lean mixture  rough idle  stalling  incorrect idle speed  If turning mixture screws does not change idle speed, idle circuit is not working properly

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System Problems Appear at very low engine speeds

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle System Inspection  Tiny bits of dirt can upset the idle circuit  Clogged idle passage  restricts fuel flow to the idle port  lean mixture and poor idle quality  Clogged idle air bleed  obstructs air for atomizing fuel  rich mixture

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Acceleration System Problems  Symptoms:  hesitation  stalling  backfiring

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Acceleration System Problems Possible causes of hesitation on acceleration

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Pump Inspection  If accelerator pump does not squirt a strong stream of fuel into the air horn as the throttle is opened, mixture will be too lean  Stream should start as soon as the throttle is moved

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only High-Speed System Problems  Symptoms:  lean air-fuel mixture (engine miss at cruise)  rich air-fuel mixture (poor fuel economy)  engine surge  Faulty mixture control solenoid or computer-control circuit may cause high-speed system problems

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Full-Power System Problems  Symptoms:  poor economy  limited engine power when passing another car

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Bad Power Valve  Diaphragm may stretch or rupture  Ruptured valve allows fuel to leak into vacuum passage  Symptoms:  very rich mixture  rough idle  poor economy

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Metering Rod Adjustment  Too far into jet:  causes a lean mixture  Too far out of jet:  causes a rich mixture

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke System Problems  Symptoms:  poor performance when cold or right after startup  black smoky exhaust  stalling  improper idle speed  Choke should close only when cold

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke System Problems Appear when the engine is started cold

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke Problems  Sticks open  the engine may not start when cold  Sticks closed  causes super-rich mixture or black smoke  the engine runs rough

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke Adjustment  Rich setting  choke too far closed  causes poor performance when engine reaches operating temperature  Lean setting  choke too far open  the engine is hard to start in cold weather  the engine stalls under acceleration when cold

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Removal  Use safe procedures to avoid fuel spills  Label vacuum hoses and wires

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only  Disassemble and clean the carburetor  Inspect parts for wear and damage  Reassemble the carburetor with the necessary new parts and gaskets  Adjust the carburetor Carburetor Rebuild

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Disassemble and Clean  Disassemble the carburetor  Remove all plastic and rubber parts  Soak the metal parts in decarbonizing cleaner  Rinse all parts  Dry with compressed air, forcing air through all passages

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Visual Inspection  Check for:  wear in the throttle body and throttle shaft  binding choke and linkage  damaged carburetor body sections  float leakage or weight  mating surface condition  damaged springs, fasteners, mixture screws, etc.

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Reassembly  Replace parts as necessary, using the proper overhaul kit  Use the number tag on the carburetor  During assembly, perform specified adjustments

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Check Float Drop Setting Distance float can move down in bowl

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Float Drop Adjustment

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Check Float Level Setting Distance from the float to the top of the bowl or air horn cover

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Float Level Adjustment Dry setting

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Choke Linkage Adjustment  Set almost fully closed when engine is cold  See service manual procedure  Adjustment may require tang bending or housing rotation

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Accelerator Pump Adjustment  Adjust so that fuel squirts into the air horn as soon as the throttle is moved off idle  Adjustments are done by bending linkage rods or arms and changing rods in their holes

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Carburetor Installation  Install using a new base gasket  Fit the carburetor on the manifold  Install the fasteners  Install all lines, hoses, and wires  Hand operate the throttle and choke valves to make sure they are not binding  Verify fuel connection before startup to avoid fire  After startup, perform final adjustments

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Fast Idle Speed Adjustment  Install tachometer  Warm engine to operating temperature  Set emergency brake and block wheels  Set fast idle screw on specified step of fast idle cam  Adjust speed to specification

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Curb Idle Speed Adjustment  Disconnect wires from idle speed solenoid  Adjust idle speed screws to specified idle speed

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only Idle Mixture Adjustment  Manual adjustment requiring a setting between lean miss and rich miss  Recheck speed after adjusting