<P> For open throttle operation a richer mixture will produce more power, prevent pre-ignition detonation, and keep the engine cooler . This is usually addressed with a spring - loaded "power valve", which is held shut by engine vacuum . As the throttle opens up, the vacuum decreases and the spring opens the valve to let more fuel into the main circuit . On two - stroke engines, the operation of the power valve is the reverse of normal--it is normally "on" and at a set rpm it is turned "off". It is activated at high rpm to extend the engine's rev range, capitalizing on a two - stroke's tendency to rev higher momentarily when the mixture is lean . </P> <P> Alternative to employing a power valve, the carburetor may utilize a metering rod or step - up rod system to enrich the fuel mixture under high - demand conditions . Such systems were originated by Carter Carburetor in the 1950s for the primary two venturis of their four barrel carburetors, and step - up rods were widely used on most 1 -, 2 -, and 4 - barrel Carter carburetors through the end of production in the 1980s . The step - up rods are tapered at the bottom end, which extends into the main metering jets . The tops of the rods are connected to a vacuum piston or a mechanical linkage which lifts the rods out of the main jets when the throttle is opened (mechanical linkage) or when manifold vacuum drops (vacuum piston). When the step - up rod is lowered into the main jet, it restricts the fuel flow . When the step - up rod is raised out of the jet, more fuel can flow through it . In this manner, the amount of fuel delivered is tailored to the transient demands of the engine . Some 4 - barrel carburetors use metering rods only on the primary two venturis, but some use them on both primary and secondary circuits, as in the Rochester Quadrajet . </P> <P> Liquid gasoline, being denser than air, is slower than air to react to a force applied to it . When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase . This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble")--an effect opposite to that which was demanded by opening the throttle . This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat . This extra shot of fuel counteracts the transient lean condition on throttle tip - in . Most accelerator pumps are adjustable for volume or duration by some means . Eventually, the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed . </P> <P> The accelerator pump is also used to prime the engine with fuel prior to a cold start . Excessive priming, like an improperly adjusted choke, can cause flooding . This is when too much fuel and not enough air are present to support combustion . For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts . </P>

What does a diaphragm do on a carburetor