Carburetion

1. Carburetion

2. Terms • Venturi – restriction in main air passage • Increases air speed thereby decreasing the static pressure • Air bleed – air passage to help atomize fuel • Jet – hole or orifice of specific size to limit fuel flow • Throttle air valve – controls volume of air-fuel mixture

3. Bernoulli's Principle • Carburetors operate because of pressure differentials • Bernoulli's principle explains the relationship between air speed and air pressure • As air speed increases, static pressure decreases and dynamic pressure increases • As air speed decreases, static pressure increases and dynamic pressure decreases

4. Bernoulli's Principle Dynamic Static

5. Venturi

6. Venturi • Carburetors use a venturi and Bernoulli's Principle to meter fuel

7. Carburetor Circuits • Float • Idle • Transition • Main • Power • Acceleration • Choke • Secondary

8. Float Circuit • Maintains correct fuel level in carburetor float bowl • Higher float level causes main circuit to start earlier • Low float level causes main circuit to start later

9. Idle Circuit • At idle, fuel is pulled from the idle circuit • Many things determine the amount of fuel delivered • Float level • Idle feed restrictor • Air bleed • Idle mixture screw • Manifold vacuum at idle • No more than .040” of transition slot should be exposed at idle

10. Transition Circuit • As the throttle is increased slightly from idle, the transition circuit begins to add more fuel to the engine • Except for the idle mixture screw the same things that affected fuel flow in the idle circuit, affect fuel flow in the transition circuit

11. Main Circuit • As the throttle is increased, manifold vacuum is decreased and the main circuit takes over fuel delivery • Flow rate is determined by main jet size and pressure differential across main jet • Pressure differential is determined by the amount of air flow through the booster

12. Main Jet Power Circuit • At high load levels, more fuel is required to produce maximum power, the power circuit provides this extra fuel • When engine vacuum drops below a pre-set level the power enrichment valve opens, allowing extra fuel to flow into the main circuit

13. Power Circuit (Edelbrock and Quadrajet) • Edelbrock and Quadrajet carburetors use a slightly different approach to fuel enrichment • Metering rods are inserted into the main jets, which causes fuel flow to be decreased. • When engine vacuum drops, the metering rods lift out of the jets increasing fuel flow

14. Accelerator Circuit • When the throttle is rapidly opened, airflow suddenly increases, however the main circuit takes a little longer to increase flow to the needed level • The accelerator pump supplements the engine with additional fuel for the split second that the main circuit needs to “catch up to engine airflow

16. Choke Circuit • When the engine is cold, fuel does not vaporize well, more fuel must be added to overcome this problem • The choke closes the top of the carburetor off from atmospheric pressure, causing the engine vacuum to pull extra fuel from the main circuit at idle Choke Plate

17. Secondary Circuit • On four-barrel carburetors, the two front or “primary” barrels are used during normal operation • During high load situations, when extra power is needed the rear two barrels or “secondaries” open and allow extra air and fuel to enter the engine

18. Additional Carburetor Systems/Components • Choke pull-off (vacuum break) • Dashpot • Hot-idle compensator • Anti-dieseling solenoid • Idle stop solenoid • Variable Venturi

19. Choke Pull-Off • The choke pull-off opens the choke plate slightly once the engine starts and vacuum is generated to lean the mixture out somewhat

20. Dashpot • The dashpot slows the closing of the throttle so that the engine doesn’t stall during rapid deceleration

21. Hot Idle Compensator • When the engine temperature rises to a pre-determined level, the HIC valve opens, allowing more air to enter the engine • This raises the idle speed which circulates more coolant through the engine

22. Anti-Dieseling Solenoid • Is energized when the ignition is on • When energized it holds the throttle blades open to the angle that maintains the proper idle-speed • When the ignition is turned off, the solenoid de-energizes, completely shutting the throttle blades and preventing dieseling • May also act as an idle stop solenoid

23. Idle Stop Solenoid • Used on vehicles with high electrical loads or air conditioning • When the air conditioner compressor engages, the extra load on the engine lowers the idle speed • The idle stop solenoid is energized whenever the compressor engages, this raises the idle speed up to the proper RPM

25. Variable Venturi • Variable venturi carburetors use moveable venturies to keep the velocity of the incoming air optimal

26. Feedback Carburetors • Feedback carburetors were used in the early to mid-1980’s to provide more precise control of the air-fuel mixture • A mixture control solenoid, operated by the ECM adjusted the fuel flow rate • The ECM used the oxygen sensor, RPM, manifold pressure, and throttle position as inputs

27. Feedback Carburetors Mixture Control Solenoid Mixture Control Rod Float Bowl Main Jet

28. Feedback Carburetors • When the mixture control solenoid is energized the metering rod is forced into the jet, reducing fuel flow • When the mixture control solenoid is de-energized the metering rod is pulled out of the jet increasing fuel flow • GM feedback carburetors use the duty-cycle principle • Dwell (time on) • Normally between 10% and 50%