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1980 91SC Backfiring and lack of power

Fuel System

Vehicle Information: Model: 911 SC; Year: 1980; Body Type: Coupe; Total Mileage: 94000;

In October my engine hesitated briefly at about 60 mph. Within 20 miles, the engine would not accelerate without backfiring, but would run at highway speeds of 60 - 70 mph once the speed was reached. The slightest hill caused the speed to drop immediately. By the time I got to the repair shop, the engine was running fine with no backfiring and plenty of power, but the throttle linkage was destroyed and the air box had separated (replaced with new throttle linkage and a new air box with a pop-off valve). When the car was returned, I had a cold starting problem (car would not start for at least 45-60 seconds), but otherwise the power was fine when warm. About two months later, the original problem occurred again with engine hesitation at 60 mph followed by massive backfiring and loss of power within 20 miles. This time the mechanic said the air box is fine, but the intake air boot was leaking (the car had been running even stronger than usual, but I suspect it was due to the leaking boot). Now, the car does not run well cold with some backfiring and great hesitation. When warm, the car starts fine, but does not seem to have much power until above 3500 rpm when it acts like a multi-valve power surge. The engine rpm when cold is about 900 rpm, but when warm will stick at about 1800 rpm (drops to about 1300 if I blip the accelerator). The warm-up regulator valve was replaced last week with a used valve. Thanks for any help or advice. The car has always run very well and been extremely reliable.

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