A choke performs the fuel mixture adjustments necessary to start a cold engine. When the fuel-air mixture is too cold, the engine won't start properly, or will stall out periodically. The choke causes the fuel air mixture to be enriched. It does this by partially blocking off the entering air. When the choke plate closes, it creates a larger vacuum which draws more fuel from the fuel bowl. This richer fuel mixture can burn at an even lower temperature and allows the engine to warm up without stalling.