Purpose of Carburetion• A carburetor’s primary purpose is to produce a

mixture of air and fuel for different engine operating conditions.

Purpose of Carburetion

• Store

• Deliver

• Filter

• Mix air and fuel

• Meter air and fuel

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

Carburetor Operating Conditions

• Cold– Choke / prime

• Low speed– Idle

• Mid speed – Idle and high speed

• Acceleration– Accelerator pump

• High speed– Main discharge nozzle

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

Carburetion Pressure Differences• Air horn

– Air enters at Atmospheric pressure

• Venturi– High vacuum is

created

• Engine side– Low vacuum

created on the intake stroke

Carburetion• Atomization

– Turning a liquid into little droplets

• Vaporization– Turning liquid

droplets into a vapor or gaseous state

Carburetion

• Types of carburetors - air direction– Side draft

Carburetion

• Types of carburetors - air direction– Up Draft

Carburetion

• Types of carburetors - air direction– Down draft

Carburetor Circuits• Float circuit

– To maintain proper fuel level

Carburetor Circuits

• Choke circuit– Provides a

rich air-fuel mixture for cold starts

Carburetor Circuits

• Throttle – Regulates

air –fuel mixture entering the cylinders

Carburetor Circuits

Idle circuit– Supplies air-

fuel mixture when the throttle is closed for low speed operation

Carburetor Circuits• High Speed

circuit– Full throttle– Maximum

air – fuel flow to develop full speed and power

Carburetor Circuits• Accelerator

circuit– Provides

additional fuel to maintain the air fuel ratio when the throttle is quickly opened.

Carburetor Circuits• Primer circuit

– Forces addition fuel into the engine prier to cold starting

Carburetor Circuits

• Diaphragm uses atmospheric pressure and vacuum to move fuel into a chamber of the carburetor to be used as needed

Carburetion / Governors

• Manual throttle controls

• Governor throttle controls– Limit low speed– Limit high speed– Maintain constant speed

Carburetor / Governor• Air vane governor

– Pneumatic

– Flywheel air pressure

Carburetion / Governors• Centrifugal (mechanical) governor

– Rotating mechanical force

Carburetion Air Filters

• Air cleaner purpose (3)– Filter air to remove dirt– Protect against back fire– Quiet engine operation

Carburetion Air Filters• Foam type air cleaners (reusable)

Carburetion Air Filters• Paper type air filters (replaceable)

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

Carburetor Adjustments• Before making any adjustments clean and

inspect parts for wear !!

Carburetor Exploded View

Running Conditions• Too Rich

– 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

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.

HAVE A NICE DAY