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Fuel System

Vehicle Information: Model: 911S; Year: 1973; Body Type: Targa; Engine (type, size, modifications): 2.4 , MFI, largely stock; Total Mileage: 100K+;

Here's one for you. I know that the fuel cutoff system on my '73 S (with MFI) works because the RPMs will "hunt" or surge up and down in the low range when I manually engage the system while holding the throttle open. With the cutoff system working, however, the car backfires often, usually when returning to the gas, either while up-shifting or after lifting to brake. If I remove the speed switch/RPM transducer, however, the backfiring stops. As I understand it, the whole idea behind the cutoff system is to avoid just the sort of backfiring that I get when the system is working. Any ideas why my system seems to have an opposite effect?

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