seafoam everything would be my next step. a little bit in your tank, a little down your stacks, and suck a little through your vacuum lines (same ones you use to sync your carbs) not all at once, try one thing first see the result and go from there. also how long did you try and crank it? its going to take a bit of time for the fuel you just drained to work its way back into the lines/bowls. maybe you gave up to early?