I went through this with my BUSA Citabria Pro with a 1.60, cowled in and no fun to get to much less see what was going on. After an hour of messing with the set up I pulled the cowl, put the prop back on and started the engine. What greeted me was a nice stream of bubbles comming from the pump to the carb, NONE from the tankt to the pump though. The only possible explanation for this would be a ruptured diaphragm in the pump letting the air through from the crankcaase.

Since I really wanted tto fly the plane that afternoon and it has the tank mounted on the CG I was stuck. In a pinch I pulled the Cline regulator off my UCD 60 a put it on the C-Pro and fired it up. I was so impressed with this set up I trashed the pump and the Cline is still on that plane.