This is caused by the Warthog HOTAS. See this FAQ to solve the problem.
The fuel always floods the engine
This usually happens in XP11 if you have a Warthog HOTAS connected to your PC. To solve this issue: Open the joystick options in XP11 Select the Warthog HOTAS Make sure that the fuel pump switch is not assigned to
How can I start the engine when it’s flooded?
Just set full throttle and close the mixture then engage the starter. As soon as you see that the prop is accelerating, set full mixture and reduce the throttle.