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The Fuel System:
Principles of Carburetion
Construction of the Basic Carburetor
Throttle Valve
The throttle valve is used to regulate the speed and power output of the engine. It is controlled by the accelerator pedal, and usually consists of a flat, round plate that tilts with the throttle shaft. As the accelerator pedal is fully depressed, the throttle valve is moved from a position of completely restricting the throat to being completely open. An idle stop screw is used to keep the throttle valve open slightly so that the engine may run at a regulated idle speed with no foot pressure on the accelerator. This screw may be turned in or out to regulate engine idle speed.
FIGURE 11. THROTTLE VALVE.
Float Circuit
Purpose
The float circuit maintains a steady working supply of gasoline at a constant level in the carburetor. This is very critical to proper engine performance. An excessively high float level will cause fuel to flow too freely from the discharge tube, causing an overly rich mixture; whereas an excessively low float level will cause an overly lean mixture.
Operation
The fuel pump delivers gasoline under pressure to the carburetor. The following events occur as the gasoline enters the carburetor through the fuel inlet:
FIGURE 12. FLOAT CIRCUIT.
(a) The gasoline begins to fill the float bowl.
(b) The float rises with the level of the gasoline.
(c) The needle valve is closed by the rising float as the fuel reaches the desired level in the float bowl.
(3) As the engine uses the gasoline from the float bowl, the level will drop. This will cause the float to drop, which will open the needle valve to let in more fuel.
(4) Venting (figure 13). The pressure in the float bowl must be regulated to assure the proper delivery of fuel and purging of vapors. The following systems and devices are added to the float circuit system to provide for these needs.
FIGURE 13. CONTROLLING FUEL BOWL PRESSURE.
Balance Tube
Due to the restrictions imposed by the air filter and changing air velocities because of varying engine speeds, the air pressure in the air horn is usually lower than atmospheric pressure. The pressure in the float bowl must equal that of the air horn in order for the carburetor to provide fuel delivery. A tube called a balance tube is run between the air horn and the float bowl to accomplish this task.
Idle Vent
Because gasoline is highly volatile, it can create overly rich mixtures during long periods of engine idle. This is because the fuel begins to evaporate in the float bowl and the vapors get into the air horn through the balance tube. The solution to this problem is to have an outside vent for the float bowl which is opened whenever the engine is idling. The idle vent is activated by linkage from the throttle valve. The idle vent system on later vehicles may be part of the emission control system. |
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Glossary
Article Pages
1. Introduction
2. Introduction To Gasoline
3. Fuel System Introduction
4. Fuel Tanks
5. Fuel Filters
6. Fuel Pumps
7. Fuel Tank Ventilation Systems
8. Intake Manifold
9. Air Filters
10. Principles Of Carburetion
12. Systems Of The Carburetor
13. Fuel Injection Introduction
14. Multiple Unit Injection Pump System
15. Unit Injection System
16. Distributor Injection System
17. Pressure Time System
18. Supply Pumps
19. NEED TO MOVE ARTICLE
20. Fuel System Inspection
21. Air Induction
22. Supercharging
23. Supercharger Systems And Pumps
24. Rotating Compressors
25. Inspection Of Compression Ignition Engine Fuel Systems
26. Inspection Of Fuel System Components
27. Inspecting A Malfunctioning Vehicle
28. Introduction To Diesel Engine Fuel Systems
29. Introduction To Diesel Fuel
30. Introduction To Diesel Fuel System Design
31. Diesel Engine Combustion Chamber Design
32. Diesel Engine Fuel Injection System
33. Diesel Engine Fuel Pumps
34. Diesel Engine Governors
35. Diesel Fuel Injection Pump Timing
36. Cold Weather Starting Aids
37. Diesel Fuel Filters
38. Diesel Engine Brake Systems
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