Guys, I have an idea....
The Carb has a float to actuate the "Needle & Seat" yeah?
As the rear tire hits the ground, the angle or the whole rear "cradle" (engine, swing-arm, wheel etc) tilts like a pendulum. As pressure is put on the rear tire, this will force the engine down, and as the carb is near the front of the engine, this will be subjected to the most movement or change in angle.
As the carb changes angle, would the movement in the float affect the fuel distribution, and maybe create a surge of available fuel to the engine
Food for thought......
Josh