Weber 44idf carbs

Vehicle Information: Model (914/4, 914/6): 914/4; Year: 1973; Engine (type, size, modifications): 2.0; Total Mileage: 85042;

I had my enging worked on and even after the mechanic finished the work setting the timing, valve adjustment, new plugs/distributor, air measurements of carbs, the engine still backfires/makes popping sound when in first and second gears especially