Yes, setting up and mounting the carbs correctly is difficult. But once done properly they are reliable and keep in tune. Only the most experienced techs know these ins and outs of these old complicated Zeniths. The factory set-up procedure is about 9 pages long. I have it in PDF if you need a copy email me. Adjusting these is not very intuitive, but I can help you with what to look for and how to set them up.
Here again the USA versions with emissions are a different animal than the emission free original versions in the 220 SB, 250S, 230 (finback) etc. In the US, timing ignition retard, tamer camshafts, timing and engine cooling temperature relays and sensors, and anti-dieseling valves were added by the late 60s and early 70s. Sizes of carb. venturis, fuel and air jets, and basic adjustment procedures changed as did emission regulations.
If you know carburetion, you just do not get into these and start turning adjusting screws. You need to have the set-up procedure and know what everything does.
Mounting is critical, the base mounting bolts become loose with age, and the paper gaskets above and below the heat insulators shrink and cause vacuume leaks. The rubber secondary diaphragms are almost always split after 20 years. The accelerator pump seals may not be working anymore, causing hesitation at acceleration. Adjusting them and cleaning the fuel discharge tubes is critical.
If you get this far the automatic chokes with an electric heating element are the subject of volumes of discussions by pre-maturely grey haired owners. Only the best most experienced can easily achieve the correct warm up idle, cold start up and hesitation free operation.
On cars which have sat for years, fuel varnish will clog up tiny passages. Complete tear down is most often needed. One idle passage can only be cleaned by drilling out a factory plug, and re-installing it after the dirt and varnish are reamed out.
Now that I have scared you a bit, these units will run well for years, once restored, mounted correctly, synchronized correctly and adjusted correctly.
Lastly, I have installed many W129, W130 mechanically injected engines in W114 cars(cioupes and sedans) mostly for the increased power, as I used this combination in my SCCA Pro Rally car back in the 80s. Its more work than getting the carbs going right. Lots of info on this if you go that route. If you do you will be building a car which may have value only to yourself.
