Is there a way to increase the rate of fuel delivery from the acceleration pump? If it's lean squirt more fuel in.

Physically what's happening is that the pressure increases in the manifold rapidly when the throttle opens so the quantity of fuel stuck to the walls increases until a new balance is achieved. So, following the lean hesitation on throttle application you get a rich hit when you close the throttle again. I don't know much of the details of your carb but increasing the accel fuel dose would probably fix it.