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

2. Carburetor Fundamentals
Chapter 24
Carburetor Fundamentals

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

4. 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

5. Bolts to the intake manifold
Carburetor Location
Bolts to the intake manifold

6. Basic Carburetor Parts

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

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

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

10. 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

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

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

13. Float controls the needle valve
Float System
Float controls the needle valve

14. 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

15. Basic Float Operation

16. 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

17. 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

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

19. 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

20. 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

21. 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

22. 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)

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

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

25. 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

26. 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

27. Accelerator Pump Linkage

28. 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

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

30. Diaphragm-Type Pump

31. 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

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

33. 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 kph) or rpm
Components:
high speed jet
main discharge tube
primary and boost venturis

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

35. 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

36. 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

37. 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

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

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

40. 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

41. 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

42. Metering Rod Action

43. 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

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

45. 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

46. Power Valve Action

47. 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)

48. Basic Choke System

49. 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

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

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

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

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

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

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

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

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

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

59. 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

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

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

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

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

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

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

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

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

68. Two-Stage Carburetor

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

70. 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

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

72. 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

73. Computer-Controlled Carburetor System

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

75. 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

76. 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

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

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