Carburetor Theory
What is combustion?
The rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat/thermal expansion
What is fuel volatility?
The likelyhood of a fuel to evaporate and for that vapor to ignite
What causes atmospheric pressure?
results of the weight of air surrounding earth
What component meters fuel flow to the idle circuit?
pilot jet
Name two safety items recommended when cleaning carburetors.
Protective gloves and safety glasses
What is the stoichiometric ratio (expressed as lambda λ = 1.0) for gasoline?
The theoretically perfect air-fuel ratio for gasoline at λ = 1.0 for most efficient combustion; balances CO/HC vs NOx/O2
What is the maximum recommended ethanol percentage in fuel?
10% ethanol (E10)
State Bernoulli's principle of fluid flow as it applies to a carburetor venturi.
when a fluid/gas flows through a narrowing, its speed increases and pressure decreases
What is the purpose of the emulsion tube?
holes progressively expose to air from high-speed air bleed as fuel level decreases to improve vaporization
Why must non-metallic parts be removed before using canned spray carb cleaner?
Spray cleaners can damage rubber/plastic; remove non-metallic parts to avoid degradation
Explain the role of the idle circuit when the throttle plate is nearly closed
Supplies fuel on the engine side of the throttle plate at low airflow so the engine doesn't stall when the throttle is nearly closed.
List two recommendations and two things to avoid regarding fuel and oil.
Do: Use fresh unleaded (minimum octane 87 at sea level; 85 at high altitude); use briggs and stratton advanced formula fuel treatment and stabilizer
Don't use ethanol blends greater than E10; don't mix oil with duel: don't store fuel over 30 days
Explain how piston movement (TDC to BDC) creates the pressure differential that draws air through the carburetor.
during intake stroke (TDC to BDC) piston creates lower pressure in cylinder versus outside atmosphere, drawing air through the carburetor due to pressure differential.
Explain the role of floats in fuel delivery.
controls fuel level in the bowl by operating the inlet needle; ensures correct fuel supply to jets
Describe one sign that foreign material has caused a carburetor problem and how identifying the material helps service.
Clogged jets, sediment, or colored deposits (blue/green indicates copper extraction). Identify materials allows for targeted cleaning/replacement.
Describe how air bleeds affect the air/fuel mixture before it enters the carburetor throat
Air bleeds provide air to the main and idle circuits to pre-atomize the air/fuel mix, improving vaporization, fuel economy, and reducing emissions.
What is vapor lock? What is one operating condition that can cause it?
Vapor lock is when vapor bubbles form due to agitation, heat, and high volatility, restricting fuel flow; caused by hot conditions or high volatility fuels.
Differentiate between internally vented and externally vented carburetor bowl vents. Give one implication of each for fuel flow.
Internally vented are bowl vents between air filter and venturi (vented into filters air path). Externally vented is a vent located on the outside air path. Implication is internal venting keeps bowl at ambient filtered air pressure and external venting exposes the bowl to outside pressure which can affect fuel flow and contamination risk.
Explain the function of transitional holes and when they are used during engine operation.
supply fuel when load increases at top speeds; allow idle circuit fuel to continue feeding until main circuit takes over
Explain why a carburetor with rust throughout should be replaced.
Water causes rusting. If rust is present throughout the carburetor, replacement is recommended because internal corrosion affects sealing and passages.
Explain why combustion "dead" areas in a combustion chamber cause increased HC and CO emissions
Dead areas are areas where the flame front cannot reach efficiently; lower temperatures lead to incomplete combustion, producing more CO and HC.
How does gasoline additives and stabilizers are used and why fresh fuel is recommended for small engines?
Used to enhance performance, reduce degradation during storage; fresh fuel avoids deposits and phase separation.
How does the throttle plate position control main-circuit fuel delivery via pressure differential at the venturi?
Its position changes airflow and venturi pressure; more open throttle increases airflow and reduces venturi pressure, drawing more fuel through main circuit.
Describe the sequence of fuel paths from the float bowl to the main nozzle at high load.
Fuel in bowl → inlet needle/float regulates level → fuel passes through pilot/main jets and emulsion tube → main nozzle into venturi. At high load, high-speed air bleed and emulsion tube holes progressively meter fuel to main nozzle
Outline the step-by-step procedure to remove and reinstall the float and inlet needle during carburetor teardown and reassembly (include torque specification for bowl nut).
Teardown/reassembly steps: Disconnect spark plug, shut fuel valve, remove carburetor, invert and remove float bowl (10 mm), observe float/needle, remove float hinge pin with pliers, remove needle, remove pilot and main jets and emulsion tube, note count of idle stop turns, inspect parts. Reassembly: reinstall pilot jet, reinstall idle stop screw same turns, install emulsion tube and main jet, install float and inlet needle, install float bowl with gasket and orient drain, torque bowl nut to 50 in-lbs, reinstall carburetor on engine