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Popping and idle jetting

Fuel System

Vehicle Information: Model: 911S; Year: 1967; Body Type: Coupe; Engine (type, size, modifications): 2.0; Total Mileage: 65000;

My car has a persistient popping/backfire when decelerating between 2900 and 3400 RPM. The car is running Webers and the valves were recently adjusted by a porsche specialist. The car is running an MSD ignition with points appropriate ignition wires. The car runs fine at idle and on acceleration, but it will give a few backfires in the RPM range mentioned above. The carbs have been adjusted by two specialists and the car runs very strong. When the carbs were being worked on we noted that the main jets and venturis are as delivered by Porsche, however the idle jets are sized at 45. Could these have been changed as the car was delivered new to Denver (i.e. high altitude)? We replaced those with 55 jets, but the car would barely run. Tried 50 jets and it still would barely run. Could not adjust air or idle mixture to get it to run. The carbs are tight with no apparent leaks (same for exhaust). Any help you can offer would be greatly appreciated.

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