Gday Guys

I have just bought a savage snipe with a 15hp merc 2 stroke (2003). the motor was running fine yesterday and did half a tank ,starting first pull everytime! I was giving it 2 pumps on the bulb with the prime pulled out then on starting/firing pushing it in on initial start and then just one pull it would start(no priming). when i came home i took the cover off and checked the plugs which were only hand tight. i set the gaps to 1mm and retighened them. I cleaned the fuel filter out and changed the oil in the leg. It will fire up with prime/choke out then ill push it in  and will run for 5 seconds then cut out!? the fuel is a week old and the motor made good power on the ocean. any ideas please ?? ive checked the tank breather and its clear!