Since this is only occuring with 1/3 of a tank of fuel or less, it is not that likely to be a pressurization issue, as you would get similiar results at 1/2 throttle.

So could it be that the vent tubing is pointing in the wrong direction?

What happens if you point the nose of the plane UP when this happens?

Could the clunk be stuck foward in the tank?

Does the tank sit slighly nose down when the plane is level?

Does the clunk NOT touch the back of the tank, which is ideal?



BTW: Steel wool will ignite with the exhaust gasses from the engine fouling up the entire exhaust system. It's not a good idea.