PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
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.
(1) 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.
(2) Operation (figure 12 on the following page). The fuel pump
delivers gasoline under pressure to the carburetor. The following events
occur as the gasoline enters the carburetor through the fuel inlet:
(a) The gasoline begins to fill the float bowl.
(b) The float rises with the level of the gasoline.