The power system of Cars Carburetor of Old Cars : The power system consists of a spring-loaded piston, a vacuum passage to the intake manifold, a valve and a restriction between the valve and the main well.
The piston determines when the valve opens and the restriction determines how much fuel will be added to the main well in addition to what the main Jet is already supplying.
The power system, controlled by manifold vacuum, provides an additional fuel inlet to the main nozzle under low vacuum conditions.
Normally, the piston is held up by high manifold vacuum but any time manifold vacuum falls below the tension of the piston spring, the spring tension forces the piston down, opening the valve to increase the fuel flow to the main nozzle.
This system is operative only when the engine demands enrichment for extra power.
Why is manifold vacuum used to control the lifting of the power piston to provide additional fuel flow to the existing air supply?
Recall the explanation of manifold vacuum ?
When the engine is running at normal operating speed and only the main metering system is supplying fuel, the rate of air flowing is determined by throttle position and how much air the pistons are able to pull in on their intake strokes.
This results in a fairly high manifold vacuum (usually above 10″ of mercury).
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What determines load on the engine?
Well, if the throttle plate is opened all the way and the engine continues running at the same speed, this allows more air to enter the intake manifold because the throttle is no longer restricting air flow.
What results ?
There is more air now in the intake manifold and hence the manifold vacuum decreases. There is less pressure difference between the atmospheric air and that flowing through the intake manifold.
Referring back to the explanation of the power system, we see that the vacuum no longer holds up the piston against the spring pressure and the valve is opened.
This allows fuel to flow which in turn enriches the air/fuel mixture producing more power from the engine even though engine speed doesn’t increase.
An example of this situation occurs when the car begins climbing a hill. Think about the last time you were driving along on the level and then began to ascend a hill.
As you began to ascend, to keep the car at the came speed, you began to step on the throttle pedal and maintained your original speed.
You also heard the engine noise change pitch and get louder. This was the result of the power system cutting in and providing more power in the cylinders.
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.