OK very simple and stupid idea but if the tank is off,are you sure it isn't tilted too far forward so the gas just isn't getting to the carbs? At this point from what you have said I would look for a leak like Pat and Ed have said and double check that your needles are seated correctly. I can't remember your whole set up but might want to open those mixture screws a little ,especially if you still have the 42s in there. It does sound like a fuel/air problem to me. Stuck/bad float. Have you left the gas on without running it and looked for leaking gas? Or used a clear tube on the bowl drain to see that the levels are up where they should be?

Make sure the tip over valve is in it's proper orientation. (with no vent tube that wouldn't matter just something to check when you reassemble.)

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