Most clunk tanks I have seen set up with too stiff a fuel line to let the clunk fall foward. Most of the time if you throttle down when going straight down the fuel in the line will be sufficient till you level out. But a low pitch prop will rev the engine more on the down line which will pump more fuel out of the fuel line. So you probably need to set the tank up for a more flexible line to the clunk, longer fuel line, or a header tank.