Re: Portable fuel polishing units
By putting a recirc. filter in the injector discharge/return line, you change the parameters of the injector return pressure ... increasing pressure in the return line due to the pressure drop of the filter when clean plus accumulating flow resistance as the filter becomes 'loaded'. Such can cause an increasing 'rich' mixture for the injectors, in comparison to a discharge line that operates at essentially atmospheric pressure.
Since your system 'works', let it alone; but, if you later experienced a much high fuel consumption, this may be the cause -- recirc. FILTER becoming plugged with corresponding higher back pressure on the injector discharge line.