I thoroughly cleaned the unit before starting it up, it's only been running for about two weeks, and the air intake is brand new. I can shut off the valve completely and there is no difference in the bubbles produced. I can also remove the air intake completely and put my finger over the hole and there is still no change. The water level is set to 1 inch above the box.
close the gate valve down a very small amount
what size is the drain line from the cup
can you post a pic of the collection cup.
if you shut off the valve, cover the airhole, and still get bubbles you have an air leak, try listening at the top of the injector to see if air is leaking in. You might need to replace teh gasket up top.
hth
matt