MERLIN SW4 PLT HAS AN AIRBORNE CONFLICT VICINITY OF BFI.

Narrative

THE EVENT TOOK PLACE DURING A FLT BTWN SEATAC AND BOEING FIELD. AFTER DEPARTING TO THE W FROM SEATAC; THE SEATAC TWR ADVISED THAT I WAS CLR OF CLASS B AIRSPACE AND FREQ CHANGE WAS APPROVED. I IMMEDIATELY SWITCHED OVER TO BOEING FIELD TWR FREQ. AFTER LEVELING OFF AT 2500 FT; I SPOTTED AN ACFT AT 11 O'CLOCK POS. THE OTHER ACFT WAS APPROX 500 FT HIGHER AND ON THE SAME COURSE. SINCE THE OTHER ACFT WAS SLOWER AND HAD THE RIGHT-OF-WAY; I DECIDED TO PASS THEM FROM THE R AS DESCRIBED IN THE FARS. DURING THIS TIME I ESTABLISHED RADIO CONTACT WITH BOEING FIELD TWR AND STATED MY POS AND INTENTIONS. THE OTHER ACFT WAS MONITORING THE SAME FREQ. AS SOON AS THEY HEARD THAT I WAS IN THE SAME VICINITY AS THEM; THEY ALTERED THEIR COURSE 90 DEGS TO THE R; WHICH CAUSED THE 2 AIRPLANES TO COME IN CLOSE PROX. EVASIVE ACTION WAS TAKEN BY BOTH AIRPLANES; THEY STARTED TO CLB AND I STARTED A DSCNT. I BELIEVE THIS INCIDENT COULD HAVE BEEN AVOIDED IF I HAD REQUESTED RADAR SVCS FOR THAT SHORT 7 MIN FLT OR IF THE FLT WAS CONDUCTED UNDER IFR. IN THE FUTURE; I INTEND ON USING THE RADAR SVCS IF AVAILABLE -- NO MATTER WHAT THE CIRCUMSTANCES.