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The power valve adds fuel enrichment at a calibrated vacuum point. Thisallows the engine to run leaner at lighter loads, for better economy,and adds fuel when the vacuum drops to the opening point under load,richening the mixture. The power valve is rated by the inches of vacuumat which it opens, and should be selected to open no sooner than2-inches Hg less than the idle vacuum in gear. A good dyno operator cantune the power valve circuit transition for the engine's intendedapplication. Repeated tests rolling into the throttle at various loadsand rpm, while monitoring the mixture will uncover any troublesomeoperating ranges.