Dispatched with the left forward main tank boost pump deferred; a B767-300 diverted to an enroute airport when fuel began to burn prematurely from the right main tank; an anomaly that would require running both engines off the one remaining left main tank boost pump.

Narrative

We were dispatched with left forward fuel boost pump 'inop' and deactivated per MM. Subsequently flight experienced significant premature main right tank fuel burn and main tank imbalance. QRH procedure called for turning off right side boost pumps and feeding both engines from the only remaining left aft fuel pump due to the placarded left forward boost pump. Due to QRH conditional statement Captain queried Maintenance about premature main fuel tank burn history on this aircraft. It was discovered that the aircraft had a significant history of this problem. Captain elected to divert for repairs prior to oceanic crossing. Concern was continuing to correct premature fuel burn would have meant continuing the flight across the Atlantic Ocean with both engines feeding from a single boost pump; an unacceptable risk. Emergency was declared and flight 'not in jeopardy' message transmitted to ATC. An amended clearance was requested and received. Flight proceeded to divert airport. Emergency was declared due to anticipated overweight landing. Flight elected to use the longest runway length beyond glideslope into the wind. Landing was made at 330;000 LBS at 200 FT/min sink rate. ARFF equipment was requested and employed to check brakes after clearing the runway. It was also discovered that the flight experienced a bird strike of the right engine during approach.