My Wren SS was shipped with a felt clunk as part of the "kit" and it was recommended to be used without a UAT.

I have also found this engine very tolerant to bubbles in the fuel line. I recently landed and found the aircraft lacking power to taxi back. When I checked the UAT was only 1/2 full and the main tank was dry. There were some bubbles in the line but the engine was still runnning. Did a normal shutdown and reduced my flight timer a little....