Remove old frequency valve or not?

Yes (that would restore it to the stock euro configuration), but you may (will probably) need to do a warm idle mixture readjustment.

If you do a search on "quasi-Lambda", you should get a few prior threads with pictures/discussion about this modification.

 

For the mixture readjustment you need some way to measure the exhaust gas constituents (which can vary from a full blown 5-gas analyzer to a working O2 sensor).

However, I need to clarify my prior post. You don't need to replace the added plumbing with another straight thru hose -- you need to eliminate the added plumbing (i.e., get the shorter stock banjo bolts and not have the "stacked" banjo ends). The added stuff is plumbed in parallel with the stock flow path (which is still there).

 

Just curious: Does it work now? Do you hear FV buzzing away?

 

That looks like a "DC Johnson" box. My car had one too ('85 Euro). You will have to remove the entire hose and hose assembly and source the correct single banjo bolts on each end.

I left the O2 sensor installed as well and patched the holes made for the wiring. It looks so much better without it and it wasn't working anyway.

There are schematics on-line and I believe Johnson's son was servicing these for a while. Not sure if that is currently true. It isn't complicated and could be rebuilt, but... I had heard from a mechanic that it can cause a bit of mid range surging. My guess is that it richens the mixture to get the cats hot enough to burn clean. Often the cleanest running engine tune will not produce the least emissions. You need higher temps in the cats so a little bit rich will achieve that.

I took all that stuff, put in a nice bag and then into the box of original stuff I keep for the car.

Franny