That's a good one. I just checked Holly's instructions for the HP 80509-1 on-line. There is no mention of a center adjustment screw like the Demon has. The only pictures I was able to pull up show the threaded casting area sealed at the bottom. The likely culprit would be the throttle plate to throttle main body gasket but that doesn't explain the vacuum at the center hole.

http://static.summitracing.com/global/images/instructions/HLY-199R10016-2.pdf