Sounds like your fuel pickup line is too stiff to "clunk" to the front of the tank. It has no problem going top to bottom, but it would need to double back on itself to go to the front. Take the tank out and hold it in a "dive" and see where the clunk is, I bet it is sitting at the back of the tank, all the way at the top.

This seems pretty common, as the most popular silicone fuel lines are really pretty thick. You may want to try to find some super-thin silicone fuel line and/or a heavier clunk. I have never been able to find good flexible fuel line at any LHS unless it was part of a tank kit already.

Originally I had posted a response about the Tower mufflers, but that problem (not enough backpressure) would manifest at the upside of the loop. This turned out to be because the Tower muffler has no baffles and was not creating enough pressure in the tank to feed the carb in a steep power-on climb. I fixed my problem by switching to an O.S. muffler.