Post on 27-Mar-2015
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Purpose of Carburetion• A carburetor’s primary purpose is to produce a
mixture of air and fuel for different engine operating conditions.
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Purpose of Carburetion
• Store
• Deliver
• Filter
• Mix air and fuel
• Meter air and fuel
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Idle Mixture screwIdle Speed Screw
Choke PlateThrottle Plate
Float
Float Bowl
Pilot Jet
Main JetPedestal
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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
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Carburetor Operating Conditions
• Cold– Choke / prime
• Low speed– Idle
• Mid speed – Idle and high speed
• Acceleration– Accelerator pump
• High speed– Main discharge nozzle
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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
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Carburetion Pressure Differences• Air horn
– Air enters at Atmospheric pressure
• Venturi– High vacuum is
created
• Engine side– Low vacuum
created on the intake stroke
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Carburetion• Atomization
– Turning a liquid into little droplets
• Vaporization– Turning liquid
droplets into a vapor or gaseous state
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Carburetion
• Types of carburetors - air direction– Side draft
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Carburetion
• Types of carburetors - air direction– Up Draft
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Carburetion
• Types of carburetors - air direction– Down draft
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Carburetor Circuits• Float circuit
– To maintain proper fuel level
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Carburetor Circuits
• Choke circuit– Provides a
rich air-fuel mixture for cold starts
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Carburetor Circuits
• Throttle – Regulates
air –fuel mixture entering the cylinders
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Carburetor Circuits
Idle circuit– Supplies air-
fuel mixture when the throttle is closed for low speed operation
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Carburetor Circuits• High Speed
circuit– Full throttle– Maximum
air – fuel flow to develop full speed and power
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Carburetor Circuits• Accelerator
circuit– Provides
additional fuel to maintain the air fuel ratio when the throttle is quickly opened.
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Carburetor Circuits• Primer circuit
– Forces addition fuel into the engine prier to cold starting
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Carburetor Circuits
• Diaphragm uses atmospheric pressure and vacuum to move fuel into a chamber of the carburetor to be used as needed
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Carburetion / Governors
• Manual throttle controls
• Governor throttle controls– Limit low speed– Limit high speed– Maintain constant speed
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Carburetor / Governor• Air vane governor
– Pneumatic
– Flywheel air pressure
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Carburetion / Governors• Centrifugal (mechanical) governor
– Rotating mechanical force
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Carburetion Air Filters
• Air cleaner purpose (3)– Filter air to remove dirt– Protect against back fire– Quiet engine operation
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Carburetion Air Filters• Foam type air cleaners (reusable)
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Carburetion Air Filters• Paper type air filters (replaceable)
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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
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Carburetor Adjustments• Before making any adjustments clean and
inspect parts for wear !!
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Carburetor Exploded View
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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
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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.