A: From what you say and that these are the original carbs says both a lot and raises questions. By halfway through the 1975 model year, the HIF4 carbs as in your car were history, so unless you can find rebuilt used ones, they are all over 40 years in service
Your reply doesn't indicate that you looked at some of the issues mentioned, so you may be missing something. That the coil works, for example, has little to do with wear in the distributor affecting the idle. The suggestion of moving to a Pertronix kit was because the kit covers a lot of the ills that come with distributor wear. Since it uses a sensor that does not rely on a mechanical gapping of the points, which could vary in a worn distributor, it evens out the spark giving you a consistent, stronger spark, better idle and less chance of missing during acceleration or at speed. A weak spark will not burn the fuel completely, giving a gas smell from the exhaust.
In the 43 years they have been in service, have your carbs ever been completely and professionally rebuilt? Even if routine DYI cleaning and maintenance were done, after all these years, there's the probability of wear in the throttle shafts and bushings in the carburetor casing, that the end seals have long since deteriorated allowing air to enter around the shafts and that the carburetor jet and needle have worn, so the throttle butterfly can be out of place and the mixture cannot be properly adjusted and more. New bushings, seals and stock or oversized throttle shafts are available, but require line-bore tool, which is costly and not something used routinely, so paying a shop to do this is the best solution.
Even the oil-filled domes at the top wear and the springs weaken after all this time. BTW, I found that going to 20W50 or even gear oil in worn dampers helps. Not sure what you're using but replacing the SU carb oil with this has been helpful.
Even without wear, if the floats have not been checked, changed or their tabs adjusted. Leaving the carbs full of fuel, even under daily use, eventually the tab on the top that actuates the float chamber valve will bend and need to be raised or the float and fuel level would be too high in the chamber and allow too much fuel into the carb, giving you both the rough idle and fuel smell. Gunk and wear on the float chamber valves also prevent them from moving, opening or seating properly, allowing too much fuel into the carb. And for the thermal compensators, there's no way to tell by looking at them if they are performing.
I don't know the source or actual condition of your spare carbs, but I'd do a thorough check of them before considering installing them. I would also suggest you send them out to be completely rebuilt before installing them. Maybe live with the rough idle a bit longer and let someone like Joe Curto (1-800-726-7878) or Apple Hydraulics (1-800-882-7753) rebuild them.
Also, when was the last time you adjusted the valves? These engines use mechanical lifters, so there is normal wear and the valve gaps have t be adjusted periodically. Too closed, there's not enough fuel getting into the cylinder or exhaust getting out and the engine runs rough.
Hope that helps. Good luck and keep me posted.
Safety Fast!
Art Isaacs