use a portable outboard tank hooked to the mechanical pump on the engine to isolate the fual system. Then if it starts running bad, squeeze teh primer bulb to add fuel presure, if that fixes it, then check the fule screen on the fuel pump or replace teh pump. You could install a fuel pressure guage after teh pump and check the same thing, but it requires a special fitting, you could make one(inverted flare fitting with a t) or order one from mercury
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