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Fuel pressure on L20E (and similar) at idle should be around 2.3-2.5 bar, increasing by about 0.5 bar as RPM rises.
The construction of the valve is extremely simple; it consists of a spring-loaded membrane that covers the return channel. At the top there is a hose connector which connects to the intake manifold via a tube, thereby ensuring an increase in fuel pressure when RPM increases.
There's not much that can break inside it; there are only 3 common issues:
1) Jamming in the closed position. Fuel pressure in the rack reaches over 4 atmospheres and cannot be released due to the jammed valve. As a result, all of this goes through the injectors into the cylinders, causing severe fuel enrichment, black smoke, and poor performance. The fuel pump will also quickly fail from trying to maintain such high pressure.
2) Jamming in the open position. The valve remains open, allowing fuel to leak directly back to the tank under lower than required pressure, leading to lean mixture issues.
3) A tear or crack in the membrane. Fuel leaks directly into the intake manifold via the vacuum hose. Severe over-enrichment, possibly preventing the engine from starting at all.

Finding an original regulator is difficult and very expensive.
Used parts are not recommended as the membrane tends to dry out over time.
The best option (and most cost-effective) is to use one from a Volga car; it has matching pressure requirements. However, since it mounts on the rail instead of a hose like in Nissan cars, you need to buy what's called an 'hose brush' with a 1/8 inch thread and an internal diameter of 8 mm. Drill an appropriate hole and screw it in using PTFE tape (the wall thickness allows for this)

And cutting off the ears can be done for aesthetic reasons

Another option is to use a tuning solution. But under no circumstances should you opt for cheap Chinese options like EPMAN; they don't maintain proper pressure, and the manometer itself might even burst. Quality versions are quite expensive.