Hesitation/Stall question

Too much timing/ not enough Transferslot opening?

Is that dishwasher hose? With the filter touching the hot head?
As long as the hot fuel is under pressure, it tends to stay liquid. But as soon as you shut the pump off, if the valve at either end leaks the pressure down, the hot fuel tends to flash into vapor, and forces the fuel to flow out the leaking end. In the case of a horizontal filter, it will act like a vapor accumulator on whatever portion cannot get out of it. It is IMO, common to see those filters only half full of liquid.
There are many different fuel molecules that make up gasoline. The most volatile ones can flash to vapor at around 90*F. The most stable ones are , IIRC around 395*F.