I'm installing the fuel tank into a Pulse XT (not yet flown) and would welcome comment.  The tank is rectangular with 3 tubes - clunk feed at the bottom of the bung, the filler and pressure fed vent at the top. It is mounted horizontally with bung to the front and clunk sitting on the tank bottom where it can move freely but cannot turn back on itself. My query is: if the aircraft is fairly horizontal all should be OK but a vertically down manoevre (stall turn, loop etc) with a half full tank will put the clunk pickup above the fuel level. Will all such antics result in an engine dead with fuel starvation at a bad time? The models at my club field don't seem aware of this and just get on with it - how? Any comments / explanations are much appreciated.

the question to ask is - How long can you sustain a vertical down-line under power before it becomes under-ground?

Most engines will run for several seconds on the fuel in the feed line,  and clunks are there to enable sustained 'unusual attitudes' such as knife edge or inverted.  I've used metal tanks with fixed (ie rigid) feeds in models capable of continuous rolls, loops etc - anything except sustained inverted, that only lasts a few seconds- maybe 5 with a diesel? 

Also the inertia of the fuel in the tank means it will take a few seconds to catch up with the plane and drop to the front of the tank, exposing the clunk.