I have the same skimmer and I get little or no overflow out of the riser. Every time it start to overflow out of the riser I open and close the air valve to clean it out. I am running it with the air valve wide open try pulling your air valve off and running it like that. Check the nipple the air valve connects to for obstruction. Also to keep the skimming consistent I had to add an auto top off system to the sump.