Did you happened to check the clunk in the tank to see if the impact moved it to the front of the tank .The engine should run as good in a downline position as any other .Its sounds like a for sure out of fuel condition .check the clunk ,lines and watch the fuel line going to the carb for fuel .I have a twist and had to put the tank right up against the top to get anywhere near the center of tank position with the engine mounted upright .
What engine are you using, prop ,fuel & plug?