Power valve In a four-stroke engine it is often desirable to provide extra fuel to the engine at high loads (to increase the power output and reduce engine knocking). A 'power valve', which is a spring-loaded valve in the carburetor that is held shut by engine vacuum, is often used to do so. As the airflow through the carburetor increases the reduced manifold vacuum pulls the power valve open, allowing more fuel into the main metering circuit.
pOWER Valve iN a four-sTRoKe enginE iT iS ofteN dESirable tO pRovide extra Fuel to THe engiNe At hiGh loads (To iNcReASe ThE poweR OutPut ANd rEDUCe engine knockiNg). a 'Power vAlvE', WHich is a sprIng-lOaded Valve in THe carburetor thAT iS held sHUt by eNGinE VACuUM, is OfteN UsEd To dO sO. As THe AIRfLoW thROUgh thE CaRBUrEtor increases the RedUceD mAnifoLd vaCUum pUllS tHE POweR valve oPEN, alLOWing morE FUEl inTo the mAin meTeRIng ciRcuiT.