Any time I saw high idle speeds it was for emission control purposes, directly or indirectly. This is not mere speculation. I got it straight from a Ford training rep at more than one Ford class for technicians. One of the first examples of really high idle speeds I saw was with the 1977/1978 Lincoln Versailles, with the Electronic Engine Control I (EEC I) system, Ford's initial foray into a fully computer controlled engine - at least in Caalifornia. The cold engine idle speed was in the 2,400 RPM range. We were told to never attempt to alter the EEC I system in an attempt to lower that cold idle speed to something more palatable, as doing so is in violation of Federal (and CA) emission control law compliance. Further, the idle speed was controlled by the EEC I system, so it was not a simple matter of twisting a screw here or there to alter the cold or warm idle speed. Apparently in order to meet emission control standards for a cold engine they had to run the engine so lean that it would/could not idle properly at a lower speed when cold, much less do so without spiking unburned hydrocarbon levels. Once the engine warmed enough for that high idle speed to drop to a more practical level things were more in tune with what we usually say, a warm idle speed that was more akin to what seemed normal.
Story time... For those who dare to care about this subject...
I recall seeing earlier engines with high idle speeds once in a while, both warm and cold. In the case of our 1969 Shelby GT500, the original Holley carb had an automatic choke that was set up to have a cold high idle of I think 1,800 RPM or so. For those GT500s with factory A/C (like ours) that created a problem. The 1969 Shelby GT500s with factory A/C were required to have both an automatic transmission, and one of only two rear axle gear ratios that were quite steep for a high performance car (either 3.0:1 or 3.25:1, ours came with 3.0:1 gears). Those two requirements were allegedly put into place to help prevent air conditioner compressor failures due to excessing engine speeds due to blowing a shift and over-revving an engine, or much higher engine RPMs when running at wide open throttle with a low rear axle gear ration (like 4.11:1 or even lower 4.3:1). Well, that was all fine and good, but the high cold idle speed of 1,800 RPM, coupled with an automatic transmission, would cause the transmission to slam into gear when shifted out of Park and into Drive or Reverse. That was nerve wracking (variant spelling to "racking") unto itself. But, apparently there were enough incidents where drivers were involved in accidents due to the high torque 428 at 1,800 RPM going into gear and lurching forward or rearward, that Ford decided to "fix" the problem by replacing the Holley 735CFM carburetor with a Holley carb with a manual choke!
We have the replacement manual choke Holley on our 69 Shelby, as well as the original auto choke carb (apparently Ford did not ask to have the original carb sent back to them). In our case, the carb runs rich enough when cold that I very rarely apply the choke manually, even on very cold mornings. It takes a moment for the engine to settle down with the choke off when cold, but once I run it in Neutral or Park for a half minute or so, while holding the engine at between 1,500 and 2,000 RPM, it idles and runs just fine. The few times I have applied the choke I find it better for me to just fully open the choke plate and leave well enough alone. Surprisingly, once warm the engine shows no indication of running too rich. No black smoke, no fouled plugs. It just idles happily at 650 RPM in Drive, just like the specs call for.
Anyway, anytime I see a high idle speed for a carb spec I remember the one Ford instructor telling us to always comply with the factory setting because we were (as state smog certified technicians) required to adhere to the legally mandated emission control settings as given by the factory specs. He would tell us that the higher speeds were that high because the engine was running so lean when cold in order to meet the emission standards, and running that lean at lower engine RPMs when cold would cause lean misfires that made the unburned hydrocarbon levels to go really high. Fun times back then, having to explain to customers why we were not allowed to smooth out their idle with a warm engine because we had to attain as low an unburned HC level as possible, and why their cold idle speeds were so high. For my own cars I would adjust the carbs to where I got good running performance, cold or warm. But, for customer cars, and trade-in cars we wanted to resell, I would comply, even if grudgingly, with emission levels (I had a tailpipe sniffer that measured unburned hydrocarbons and Carbon Monoxide), yet run as soothly or normally as possible.
If there is any good reason for moving to aftermarket fuel injection and computer controlled fuel delivery, it is for gaining excellent performance from an engine, cold or warm. The other reason is for the ability of the computer system to properly adjust the air-fuel ratio for a vehicle that is changing altitude frequently, where in higher altitudes the engine carburetors are typically too rich.
