Just a thought...

Make sure that you have not accidentally switched the connections between the pressure line and the fuel line on the fuel tank. I'm assuming you have a two line setup.

With the engine canted and the muffler hanging low, it is a good recipe for siponing back from the tank especially if it was drawing of the fuel line. The muffler is probably large enough to hold quite a bit of fuel and when you tip it up it may dump out quite a lot.

As far as the quitting after 3 seconds goes, the symptoms you describe sound like flooding which is curious. However if the engine RPMs raise for a few seconds and then slow to a quit, it is probably a leaning issue. Again if the lines were switched it would probably suck air when tilted and starve for fuel (lean) and quit, which is similar to what you describe.

You might also check that your pressure line inside the tank curves up almost to the 'roof' of the fuel tank. If you accidentally mounted the tank upside down it may not pressurize well and may also cause the symptoms you describe.