The Frequency Valve is how the Jetronic Computer regulates Mixture (Duty Cycle)by alowing or not alowing fuel back the the tank via the Fuel Distributor circut. NJScirGTX Steve Toughill -- To subscribe or unsubscribe, send email to scirocco-L-request@privateI.com, with your request (subscribe, unsubscribe) in the BODY of the message.