I answered another post earlier like this, so i'll repeat it for this
Well were all coming up with different ideas
Ive got three for the mix
1 ) You are developing vacuum in your tank as the fuel is used up, maybe a trapped vent line. To check for this run the bike with the fuel cap loose and if its ok check for trapped vent line.
2 ) Faulty vacuum diaphragm in fuel tap (Petcock), Try running in Prime position, if its ok sort out vacuum diaphragm, dont continue to use prime position as a fix though!
3 ) If the above dont fix it, maybe ypou fuel filter is blocked, To check remove the fuel line to the carbs, switch to prime position, there should be a good flow of fuel. (Definitely a no smoking operation)
Good Luck