Originally Posted by timw
Despite the engine being able to idle, underload it cuts out.
However, on linking the engine up to a portable tank, it runs fine.
Sounds like it *could* be the fuel pump in the engine (unlikely, though, if it runs fine on the portable.)
One thing to try is to leave the filler cap open a bit to bypass your breather tube.
Does the fuel bulb on the primary system collapse when you're running? If so, it points more to fuel line clog.
Oh, and you might want to change out the filters again (I'm assuming you've got a cannister type filter in-line somewhere.) The on-engine fuel filter should be relatively cheap.