AFTER AN ANNUAL INSPECTION THE ACFT WAS PREPARED FOR A TEST FLIGHT. THE PILOT LOADED 3 PASSENGERS AND BEGAN THE RUN-UP CHECKS. DURING RUN-UP THE LEFT ENGINE RAN ROUGH. AFTER THE ROUGHNESS CLEARED THE PILOT INITIATED THE TAKEOFF. AS THE ACFT ACCELERATED DOWN THE RWY THE LEFT ENGINE RAN ROUGH AGAIN AND THE TAKEOFF WAS ABORTED. THE PILOT MADE ANOTHER RUN-UP CHECK AND ANOTHER TAKEOFF ATTEMPT WAS MADE WITHOUT INCIDENT. AS THE AIRCRAFT CLIMBED THRU 300 FEET MSL THE RT ENGINE BEGAN TO SPUTTER. THE CLIMB WAS CONTINUED TO 500 FEET MSL BEFORE THE POWER FROM THE RIGHT ENGINE WAS REDUCED AND THE PILOT SHUT THE ENGINE DOWN AND FEATHERED THE PROPELLER. THE ACFT DESCENDED AS THE PILOT INITIATED A TURN TOWARDS THE AIRPORT AND IMPACTED THE MARSHY GROUND 2.75 MILES FROM THE AIRPORT. POST ACC EXAM REVEALED WATER AND RUST PARTICILES ON THE INJECTOR DIAPHRAGM AND ON THE FUEL INLET SCREEN. AN AD WAS ISSUED ON THIS ACFT REQUIRING DAILY AND POST REFUELING CHECKS OF THE FUEL SYS BY DRAINING TO AVOID THE ACCUMULATION OF WATER AND SEDIMENT.