Modern Automotive Technology PowerPoint for by Russell Krick Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois

Carburetor Fundamentals Chapter 24 Carburetor Fundamentals

Contents Basic carburetor Basic carburetor systems Carburetor devices Carburetor vacuum connections Carburetor barrels Carburetor size Variable venturi carburetor Computer-controlled carburetors

Basic Carburetor Mixes air and fuel in correct proportions When engine is running, downward-moving pistons on the intake stroke produce vacuum in the intake manifold Air rushes through the carburetor into the engine Carburetor meters fuel and mixes it with the air

Bolts to the intake manifold Carburetor Location Bolts to the intake manifold

Basic Carburetor Parts

Body Cast metal housing contains cast and drilled passages for air and fuel Contains the main discharge tube, fuel bowl and venturi

Air Horn Also called the throat or barrel Routes outside air into the intake manifold Contains the throttle valve, venturi and discharge nozzle

Throttle Valve Butterfly valve in the air horn Controls air and fuel flow through the carburetor Controls power output

Venturi Produces vacuum 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

Basic Carburetor Systems High speed Full power Choke Float Idle Off-idle Acceleration

Float System Maintains the correct fuel level in the fuel bowl Components: float fuel inlet valve (needle and seat) bowl vent

Float controls the needle valve Float System Float controls the needle valve

Operation Float rides on the fuel Float forces the fuel inlet valve (needle and seat) closed when the fuel level reaches specified level When the fuel level drops below specification, the float rides lower, allowing fuel pump pressure to force the needle off its seat, filling the bowl

Basic Float Operation

Bowl Vent Bowl vent is used to vent the fuel bowl to atmosphere or the air cleaner Pressure acting on the fuel in the bowl must remain constant to deliver a constant air-fuel mixture ratio

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

High vacuum pulls atomized fuel out of the idle port Idle System High vacuum pulls atomized fuel out of the idle port

Operation Low Speed Jet 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 Restriction in the idle passage Limits fuel flow in the idle circuit

Idle Air Bleed Idle Passage Adds air bubbles to fuel Helps break up or atomize fuel Aids evaporation of fuel Idle Passage Carries atomized fuel to the idle screw port Idle screw port is the opening in the air horn below the throttle valve

Idle Mixture Screw Allows adjustment of the opening in the idle port Turning screw in reduces hole size, leaning mixture Turning screw out enlarges hole size, enriching mixture

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 (main discharge)

Off-Idle System Operation As the throttle is opened slightly, system feeds extra fuel to the increased air flow

Carburetor Bottom View Shows the idle mixture screw tip, idle port opening, and off-idle port

Acceleration System Provides extra fuel when changing from idle circuit to high-speed circuit (main discharge) operation Components: accelerator pump pump check ball and weight nozzle or jet

Squirts fuel into the air horn every time the throttle is opened Acceleration System Squirts fuel into the air horn every time the throttle is opened

Accelerator Pump Linkage

Operation Operates when the accelerator is pressed Accelerator pump forces fuel from the pump reservoir Pump pressure closes the pump check ball Fuel flows toward the pump check weight forcing it off its seat Fuel squirts into the air horn

Accelerator Pump Develops pressure to force fuel out of the pump nozzle into the air horn Two types: piston diaphragm

Diaphragm-Type Pump

Pump Check Ball and Weight Check ball prevents fuel from flowing back into the fuel bowl when the pump is actuated Weight prevents fuel from being siphoned into the air horn by venturi vacuum

Pump Jet Fixed size orifice Meters fuel flow out of the pump

High Speed System Also called the main metering system Supplies the engine’s air-fuel mixture at cruising speeds Functions from about 20-55 mph (32-90 kph) or 2000-3000 rpm Components: high speed jet main discharge tube primary and boost venturis

Venturi vacuum pulls fuel through the main discharge High Speed System Venturi vacuum pulls fuel through the main discharge

Operation When airflow is high, venturi vacuum pulls fuel through the main jet, through the main discharge tube, to the discharge nozzle into the air horn Atomization takes place when the air bleed introduces air into main discharge tube

High Speed Jet Main Discharge Tube Calibrated size hole in a removable insert Hole size determines how much fuel flows through the circuit Main Discharge Tube Carries fuel from the fuel bowl to the discharge nozzle Air bleed adds air to atomize fuel

Primary and Booster Venturis Primary venturi is formed in the side of the air horn produces vacuum at high air flow Booster venturi is an airfoil-shaped ring in the air horn produces vacuum at low air flow

Full Power System Enriches mixture for high-speed, high-power conditions Components: metering rod linkage vacuum control or computer-controlled solenoid power valve

Full Power System When the throttle is opened, the metering rod is lifted out of the main jet

Operation Tapered metering rod moves in and out of the main jet Controlled by manifold vacuum in relation to engine load or through mechanical linkage controlled by the throttle May be driven by a computer-controlled solenoid

Operation When metering rod is low, deep into the jet, flow is restricted, lean mixture results When metering rod is raised at high load, low vacuum, flow increases, rich mixture results

Metering Rod Action

Power Valve Provides high-speed fuel mixture Serves as an extra jet, feeding fuel into the high speed circuit Components: fuel valve vacuum diaphragm spring

Serves the same function as the metering rod Power Valve Serves the same function as the metering rod

Operation Spring holds valve open Vacuum acts on diaphragm to close the valve When load is high, manifold vacuum is low, spring pushes the valve open, enriching mixture, increasing power When load is low, high manifold vacuum acts on the diaphragm, forcing valve closed, leaning mixture

Power Valve Action

Choke System Supplies rich air-fuel mixture to aid cold engine starting and running Components: choke plate (butterfly valve) thermostatic spring (bimetal) vacuum piston (choke break)

Basic Choke System

Cold Engine Operation When cold, bimetal spring closes the choke At startup, high vacuum below the choke pulls a large amount of fuel out of the main discharge Vacuum piston (choke break) cracks the choke open a small amount to prevent flooding

Hot Engine Operation When the engine warms, heat relaxes bimetal spring tension Airflow and gravity open the choke valve, leaning the mixture

Choke Heat Sources Integral and nonintegral Electric assist engine heat Electric assist hot air and electric heating element All electric two stage heating element

All Electric Choke One stage warms the thermostatic spring when cold When the engine is partially warm, both heating stages function

Opens the choke when the throttle is fully opened Choke Unloader Opens the choke when the throttle is fully opened

Carburetor Devices Fast idle cam and solenoid Throttle return dashpot Hot idle compensator Altitude compensator Vacuum connections

Increases idle speed when choke is closed Fast Idle Cam Increases idle speed when choke is closed

Fast Idle Solenoid Holds throttle open when the engine is running Allows throttle to close when turned off Avoids run-on or dieseling

Adjust fast idle speed by turning plunger Fast Idle Solenoid Adjust fast idle speed by turning plunger

Throttle Return Dashpot Causes throttle plate to close slowly, preventing stalling

Hot Idle Compensator Prevents stalling or rough idle at high temperatures Temperature-sensitive valve admits extra air increasing idle speed Compensates for low air density at high temperatures

Altitude Compensator Changes air-fuel mixture with changes in altitude Uses an aneroid: bellows device expands and contracts with changes in atmospheric pressure

Carburetor Vacuum Connections Operate various components using carburetor vacuum connections: E.G.R. valve distributor vacuum advance charcoal canister choke break

Manifold Vacuum Supplied by ports below the throttle plate Receive full intake manifold vacuum Powerful signal at idle

Ported Vacuum Supplied by ports above the throttle valve Receive vacuum only when the throttle valve is opened No signal at idle

Position of the port determines when vacuum is present Vacuum Ports Position of the port determines when vacuum is present

Carburetor Barrels Primary Secondary operate under normal driving conditions Secondary function under high engine power output opened by secondary diaphragm

More barrels are used on larger engines Carburetor Barrels More barrels are used on larger engines

Primary/Secondary Operation Secondary barrel opens when engine power output is high

Two-Stage Carburetor

Carburetor Size Stated in cubic feet of air per minute (CFM) Determines the amount of air that can flow at wide-open throttle

Variable Venturi Carburetor Adjusts the diameter of the venturi to maintain constant air speed Piston slides in and out to regulate size Used on some imported cars and motorcycles

Slide-Type Variable Venturi Slide moves in and out to control fuel and airflow

Computer-Controlled Carburetors Computer calculates mixture requirements from various inputs Controls mixture using a solenoid-operated valve in the carburetor Controls idle speed with an electric motor and gear mechanism which moves the throttle plate

Computer-Controlled Carburetor System

Mixture Control Solenoid Opens and closes quickly to change air-fuel ratio

Operation Mixture is controlled by cycling the mixture-control solenoid several times a second Computer varies the on-time (duty cycle) to vary the mixture

Rich Command Lean Command Solenoid is off more than it is on When de-energized, a spring forces the valve open for more fuel flow Lean Command Solenoid is on more than it is off When energized, magnetism pulls the valve closed for less fuel flow

Mixture Control Solenoid This design opens and closes an air passage to control mixture ratio

Mixture is rich when the solenoid is off Solenoid Control Mixture is rich when the solenoid is off