Have you checked that you may have some trash in your main needle-seat area-what makes me think this is I have had a harley with carb problem do the same thing-if it sat for a while what would happen is some gunk from it having sat so long would get sucked up into the needle seat area and run super rich and leak all over-an hour later you could fire it up and it would run fine-pulled the bowl off and checked the needle and sure enough there was a tiny bit of crud that would hold the needle off of the seat and then float away-if the needle is viton(rubber) tipped it may well have a shred missing or similar-maybe worth a look-seeing how the vacum checked out and your smoke is white then I would think it would definitely be a carb issue-might be a situation of spending the few bucks for fresh gaskets well worth it- good luck
:cheers: