I noticed a similar condition on my 750. After all my resurrection work on the tank and carbs, I installed transparent fuel lines, at least for the time bing. What I noticed is, that when I turn on the petcock, a certain amount of fuel always starts flowing to the bowls. You would think that having just run it the day before, for example, there should still be full bowls and fuel in the lines. I'm not sure if it evaporates from the bowls, if so how much, if there is a tickle leak (don't see anything though) or what, but perhaps you have the same condition. Do you turn on the petcock and wait a few seconds before you try cranking? If not, might be worth a try.