1. Purpose of Carburetion
A carburetor’s primary purpose is to produce a mixture of air and fuel for different engine operating conditions.

2. Purpose of Carburetion
Store
Deliver
Filter
Mix air and fuel
Meter air and fuel

3. Idle Mixture screw
Idle Speed Screw
Throttle Plate
Choke Plate
Float
Pilot Jet
Float Bowl
Main Jet
Pedestal

7. Carburetion Air-fuel Requirements
Ratio of air to fuel.
Stoichiometric ratio of 14.7:1
Example 14.7 lbs of air to 1 lb of fuel
Fuel ratios
Rich / lean

8. Carburetor Operating Conditions
Cold
Choke / prime
Low speed
Idle
Mid speed
Idle and high speed
Acceleration
Accelerator pump
High speed
Main discharge nozzle

9. Carburetion Pressure Differences
Vacuum
Pressure less than atmospheric pressure
Atmospheric pressure
Force of air on a surface at 14.7:1psi
Venturi principle
A restriction in an air passage which causes an increase in air speed and a decrease in air pressure

10. Carburetion Pressure Differences
Air horn
Air enters at Atmospheric pressure
Venturi
High vacuum is created
Engine side
Low vacuum created on the intake stroke

11. Carburetion Atomization Vaporization
Turning a liquid into little droplets
Vaporization
Turning liquid droplets into a vapor or gaseous state

12. Carburetion
Types of carburetors - air direction
Side draft

13. Carburetion
Types of carburetors - air direction
Up Draft

14. Carburetion
Types of carburetors - air direction
Down draft

15. Carburetor Circuits
Float circuit
To maintain proper fuel level

16. Carburetor Circuits Choke circuit
Provides a rich air-fuel mixture for cold starts

17. Carburetor Circuits Throttle
Regulates air –fuel mixture entering the cylinders

18. Carburetor Circuits Idle circuit
Supplies air-fuel mixture when the throttle is closed for low speed operation

19. Carburetor Circuits High Speed circuit Full throttle
Maximum air – fuel flow to develop full speed and power

20. Carburetor Circuits Accelerator circuit
Provides additional fuel to maintain the air fuel ratio when the throttle is quickly opened.

21. Carburetor Circuits Primer circuit
Forces addition fuel into the engine prier to cold starting

22. Carburetor Circuits
Diaphragm uses atmospheric pressure and vacuum to move fuel into a chamber of the carburetor to be used as needed

23. Carburetion / Governors
Manual throttle controls
Governor throttle controls
Limit low speed
Limit high speed
Maintain constant speed

24. Carburetor / Governor Air vane governor Pneumatic
Flywheel air pressure

25. Carburetion / Governors
Centrifugal (mechanical) governor
Rotating mechanical force

26. Carburetion Air Filters
Air cleaner purpose (3)
Filter air to remove dirt
Protect against back fire
Quiet engine operation

27. Carburetion Air Filters
Foam type air cleaners (reusable)

28. Carburetion Air Filters
Paper type air filters (replaceable)

29. Carburetor Adjustments
Before making any adjustments clean and inspect parts for wear !!
Rough adjustment
Fine adjustment
Idle mixture adjustment
High speed mixture adjustment
Rpm adjustment

30. Carburetor Adjustments
Before making any adjustments clean and inspect parts for wear !!

31. Carburetor Exploded View

32. Running Conditions Too Rich Too Lean Just Right
Air-fuel mixture with excessive fuel
HC and CO pollution
Exhaust black soot
Too Lean
Air-fuel mixture lacking fuel
CO and NOX pollution
Engine overheating
Just Right
Smooth engine operation
Rapid Acceleration
Low pollution / Clean Exhaust
Normal operating temperature

33. Carburetion
The purpose of the fuel system is store, deliver, mix and meter air and fuel for the engine to function over a variety of operating conditions.