Sadly I am having the same problem with JF Viscount. I have flown the model from Calclassics that even though is free it is much more accurate regarding fuel system management. I love accuracy in flying.
This is how it works in this plane:
To avoid high impact loads on the carriage during landings it is mandatory that the tanks with more capacity (in FSX there are Left Aux and Right Aux with 792 gls each) be used first during cruise then when the four tanks (there are Left Tip and Right Tip tanks with 348 gls each) have the same quantity to land with more or less the same quantity on all four tanks.
To accomplish that there are two actions to carry out once in cruise level in this order:
1. Switch open the left (Port) and right (Starboard) switches at the pedestal. Don't touch the center one. (In the JF Viscount like Hangar 200 said there is a hand but does nothing)
2. Switch close both fwd and aft valve at both tip tanks.
3. With this configuration the engines are being fed by the two Aux tanks.
4. When is time to go back to normal for descent and landing first switch the tank valves to open and then close the two fuel crossfeed switches at the pedestal.
It is a real pity that JF has done nothing about this mistake but there are also other errors like:
1. The Master Start/Ignition switch does not move even with the little hand active so it always is on which is wrong.
2. There is no ground power available.
3. Too many switches not working even the little hand is active
4. Even it says that there is a way to expedite the fuel trimmer switches I could not find it.
5. No cabin pressurization control knob to adjust the cabin pressure. Only the spill valves.
6. The manual is incredibly simple for this wonderful aircraft be enjoyed at full.
So, I will continue flying my free version and until JF fixes all these issues, I will consider my 30 bucks in a garbage recipient.