This is a stange one. I borrowed a HP 15 Mercury (1989) from a friend. It was having a hard time running, so I replaced the Spark Plugs, put in new qaskets in the Carb, cleaned out the Fuel Filter. Now, what is happening, is that when I connect the External fuel tank, the engine will quit after about 5 seconds. The engine will run, without the external fuel tank for about 1 minute without having to connect the external tank and re-prime. I have checked the external fuel tank, and it appears to have free flowing of gasoline. Any thoughts? Your help is appreciated!