Hi Uli,

Welcome to the world of jets! Your CAT (its not a UAT) is drawing air in somewhere. The first thing to check is the cable ties round the black tube between the main tank and the CAT. These are a bad idea as there can be a small gap where the ends of the tie meet which can allow air to be drawn in between the OD of the solid tube and the ID of the flex tube. Replace these (and I would do the outlet side of the CAT too) with stainless tie wire wrapped twice round the tube before twisting off.

Next thing to check is your clunk and clunk line inside the main tank. Is it a felt clunk? There is no problem with these provided they are clean and you use well filtered kero (fit a couple of filters in your fill line from your kero bottle outside the model) but if they get dirty they clog and the pressure drop across them will accentuate any air leakage into the clunk tube and the tubing from the main tank to the CAT.

Lastly I would check with Marc at GBR Jets that it is OK to mount the CAT vertically. It may be that your air margin is reduced by doing this as the pleated element in the CAT may uncover sooner. Marc will let you know.

Also I wouldn't personally have a shutoff valve on the suction side of my pump. This is another source of air into your fuel system but it isn't causing the air in the CAT. I would re-fit it on the pressure side of the pump - but you must remember to open it before starting!!!

Hope this helps.

Malcolm