ON SWITCHING TANKS FOR LNDG; SMA HAS ROUGH ENG. SWITCHES BACK.

Narrative

I FLEW FROM BADER FIELD; NJ (AIY); TO MANASSAS; VA (HEF). APPROX 15 MI SE OF MANASSAS ARPT; IN PREPARATION FOR LNDG; I SWITCHED FUEL TANK TO THE TANK WHICH INDICATED THE MOST FULL. APPROX 3 MINS LATER THE ENG SPUTTERED AND I BEGAN LOSING ALT. I RAN THE AUX FUEL PUMP WHICH IS A HAND WOBBLE PUMP; WHICH HAD NO EFFECT. I SWITCHED BACK TO THE ORIGINAL FUEL TANK AND THE ENG RETURNED TO NORMAL OP. NOT KNOWING THE CAUSE AT THIS TIME; OR WHETHER IT WOULD RECUR; I CALLED MANASSAS TWR AND NOTIFIED THEM THAT I HAD A ROUGH ENG AND REQUESTED A STRAIGHT IN APCH. THEY REFERRED ME TO IAD FOR RADAR VECTORS. IAD APCH ASKED IF I NEEDED ASSISTANCE. I DECLINED AND TOLD THEM I HAD SOLVED THE PROB. I LANDED AT MANASSAS WITHOUT FURTHER INCIDENT. UPON EXAMINING THE ACFT AFTER LNDG; I DISCOVERED THAT THE GAS CAP WAS MISSING ON THE L MAIN TANK; AND THE FUEL IN THAT TANK HAD BEEN SIPHONED OUT COMPLETELY. I HAD APPROX 1 HR FUEL LEFT IN OTHER TANKS. SUBSEQUENT CALLS TO BADER FIELD MAINT PERSONNEL INDICATED THAT THEY HAD FOUND THE MISSING GAS CAP ON THE GND AT BADER. I DON'T REMEMBER THE DETAILS OF MY PREFLT; HOWEVER; I HAD CHKED EACH TANK FOR FUEL QUANTITY AND APPARENTLY LEFT THE L MAIN G CAP ON THE TOP OF THE WING.