Post by Scoutpilot on Jan 9, 2015 10:19:47 GMT -5
The accelerator pump circuit performs two functions. These include additional fuel during starting and additional fuel when accelerating quickly.
During the engine starting operation, the intake vacuum is quite low. Therefore, neither the low-speed nor the high-speed circuits can supply an air-fuel mixture. When depressed, before starting the engine, the accelerator pump supplies a small amount of fuel to the intake manifold where the vapors are pulled into the cylinders during the cranking operation. This, along with the choke circuit (described later), assists in engine starting.
When the throttle is opened the high-speed circuit reaction time doesn’t react as quickly as required. The accelerator pump provides increased fuel to the throat of the carburetor during these critical times.
Operation of the accelerator pump is a simple pump circuit consisting of a spring-return cylinder, two check valves, a discharge nozzle and spring to prolong the flow.
When the throttle is opened the cylinder rod is forced down forcing fuel in-between the check valves. The intake check valve closes and the exhaust check valve opens allowing fuel to flow up to the discharge nozzle in the carburetor throat. When the throttle is opened more quickly than the circuit can discharge fuel, the spring acts to prolong the fuel flow. This precisely what the high-speed circuit requires due to it’s relative slow reaction time.
When the throttle begins to close the cylinder reverses it’s movement, closing the exhaust check valve and opening the intake check valve. Fuel then flows from the bottom of the bowl through a filter screen to recharge cylinder for the next operation.