I rebuilt my carby on my XF myself, full tear down and put back together, I set everything on it according to the gregorys specs, the float was a problem had a small leak so would sink to the bottom and flood the motor, once i soldered it and set it, all was fine, I did clean the fuel filter at the same time by pulling out the centre unit in the fuel tank as well.
Maybe you are low on fuel and you have water in your petrol as well ??
I did put something here, but it vanshied, I think
"So long and thanks for all the fish!"