The ABCs of Carburetion

UM-900 (1959)

 

Page 12 of 19

CARBURETOR SYSTEMS Continued
PART THROTTLE SYSTEM
During Part Throttle operation, the lowest pressure point is in the venturi, and fuel flows into the carburetor bore through the main metering system. The idle passages are now really idle, since they are no longer exposed to the highest vacuum. This is the real economy range, where the venturi theory is in full swing and the carburetor can give the engine the best mixture it can handle.
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POWER SYSTEM
For Power, a spring loaded vacuum piston is released to open a power valve and supply the needed extra fuel. Vacuum controls the power piston operation, since it is most sensitive to load. When the load is light, vacuum is high and the piston is held up against the spring tension; when load increases, vacuum drops, the spring tension overcomes the vacuum pull on the piston and the power valve is opened.
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ACCELERATOR PUMP SYSTEM
The Accelerator Pump supplies that extra squirt of fuel needed to prevent hesitation dur­ing the change to a wider throttle opening or higher speed. Most pump plungers are spring-connected, so that when the throttle is opened quickly the spring takes up the quick motion and moves the plunger more gradually, so that the change in speed can be made more smoothly.
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