DURING CRUISE FLT, THE CFI MOVED THE FUEL SELECTOR FROM THE LT TO THE RT POSITION. THE ENG RPM THEN BEGAN FLUCTUATING. HE MOVED THE FUEL SELECTOR BACK & THE ENG BEGAN OPERATING NORMALLY BUT BEGAN LOSING POWER. WHILE DESCENDING DURING THE DARK NIGHT FOR A FORCED LANDING, THE ACFT COLLIDED WITH A BLACK NET COVERING A NURSERY & POLES SUPPORTING THE NET. EXAM OF THE COCKPIT REVEALED THAT THE FUEL SELECTOR WAS IN THE OFF POSITION. IT WAS NOT MOVED BY FIRE OR POLICE DEPT PERSONNEL. EXAM OF THE ACFT REVEALED THAT THE RT WING FUEL TANK FLEXIBLE LINE WHICH ATTACHES TO THE FUEL TANK, WAS PARTIALLY BLOCKED BY CONTAMINANT. THE LINE WAS MANUFACTURED IN 1969. ALSO, THE RT WING FUEL TANK & CARBURETOR FINGER SCREENS WERE PARTIALLY BLOCKED BY CONTAMINANT. ACCORDING TO NOTE NO.18 OF THE PIPER INSP CHECKLIST, THE FLEXIBLE FUEL LINE IS REQ'D TO BE REPLACED WHEN THE ENG IS OVERHAULED, WHICH OCCURRED IN 1984. THE CARBURETOR BOWL WAS DRAINED WHICH REVEALED THAT IT CONTAINED LESS THAN APPROXIMATELY 3 OUNCES OF FUEL. THE PAX STATED HE MOVED THE FUEL SELECTOR WITHOUT THE KNOWLEDGE OF THE CFI & HE WAS AT THE CONTROLS AT THE TIME OF THE LOSS OF ENG POWER.