I have a 50 hp Mercury OB 2-stroke, mid-seventies vintage. When I first start it up it runs fine until I stop the engine. Second start and each time afterwards, it has trouble starting. After choking, it will start but put into gear, it dies, again and again. 10 - 15 tries to get underway. Fuel line is clear, no blockage in fuel filter, bulb is hard. Fuel is mixed properly. This engine has been rebuilt but I do not know the extent of the rebuild. I have owned it about 2 months. I have to believe this may be operator error as I had a 70 hp Johnson that behaved exactly the same way. Any suggestions?