Generator engine stumbling or missing

I had a 15KW Milbank (B&S) generator Model 040274A installed by a contractor in late December 2012. When it runs under load or during its weekly run, it sound like it is stumbling or missing. The generator fuel line is directly off the meter outlet (outside) and 8 feet underground from the meter (1.5 inches in diameter) and reduced to 3/4 inches for the gas valve, sediment trap and into the flexible fuel hose at the generator with three 90 degreee turns. See pipture. The gas meter is rated at 400/308 CFH at 1/2 inch WC. Downstream from the meter on a different gas leg is a gas hot water heater and furnace both with 1/2 gas lines. The contractor listen to the generator and says that this is how all of them sound that he has installed. Is this normal?