In the USA we are free to make this type of modification with only a log book entry and suitable testing. Things may be different where you live so please check before trying anything like this.
The basic premise is that the Spanish selector/gascolator is removed along with all of the levers, linkage and mounting hardware. A new, more modern gascolator is mounted low on the engine side of the firewall and a brass three-way selector valve is mounted approximately in the location where the original push-rod passed through the firewall. This avoids having to run fuel lines all the way to the rear cockpit and back. The finished installation looks something like this:
There are two important things to note here: Both the valve and the gascolator are mounted to the steel fuselage structure, not just to the firewall. This is necessary to ensure there is no relative movement and that metal fuel lines are safe. Your installation may be different depending on where you choose to route the fuel lines through the firewall.
Next, and this is the key to the whole scheme, the original push-pull fuel control rod is modified by removing the screw that prevents its rotation (hidden behind the rear cockpit throttle quadrant) and adding two 10mm universal joints using roll-pins (or other hardware) to prevent slipping.
Now "Gravity" or "Flop tube" operation is selected not by pushing/pulling the control but by rotating it through 90 or 180 degrees.
This schematic should make it clear. Contact me if you need more details or part numbers.
--oOo--
Almost all CASA/ENMA fuel selectors seems to leak a little but this one leaks more than most and is obviously not safe to use. Does anyone have a manual, diagram or some advice on servicing these valves?
Thanks - Steve