In the air or on the ground? On the ground the fuel will run forward and there will be no fuel available, especially if you installed thicker lines and the clunk is staying to the rear of the tank (where it's supposed to stay).

When I started running Norvel engines in my combat planes, I began having the same problem. It was frustrating at first, but then (long story short) what was actually happening was the thin chincy fuel line supplied by the tank manufacturers was sucking flat and closing off. I replaced the clunk line with the same stuff I use for external line and have not had a problem since.