The spring keeps a little tension on the float arm so going over bumps does not make the float go wild. Sometimes carbs catch real hell when they are shipped. I bought a brand new Holley back in 1978 from a local parts store. I installed it, left the air cleaner off and cranked the engine for about 30 seconds to fill the bowl. All of a sudden fuel started shooting out of the front vent tube straight up! Think it shot about 2 feet into the air and hit the hood. Took the front bowl off and found the float arm was jammed in the full up position, which I assumed was from rough shipment. It can happen!