The 1978 models had to meet EPA emissions standards to be imported and sold. The standard was low hydrocarbon emissions ( unburned fuel) at idle. To achieve this, the exhaust had more back pressure, to retain some hydrocarbons to be reburned on the next fire cycle, and idle circuits that could be peak leaned. The last thing on the tune up list was to set the pilot screws at their leanest settings, for minimum hydrocarbons out the tail pipe. Exhaust sniffers were not common shop equipment in the day. So Honda printed what they found this setting to be on most of their examples and put that in the tune up manual. While the lean burn idle satisfied the EPA, Mechanical slide carbs Will go lean with every throttle lift to increase engine speed. That is why accelerator pumps had to be added, to make throttle response acceptable. And, why carbs had to be “anti tamper “ with pressed in pilot jets and limited availability of alternate sizes so owners couldn’t easily undo what the EPA mandated was best for the general public.
In the 550 manual, the final procedure was to use a finely sensitive tach, and turn in each IMS for peak RPM, while readjusting idle speed to lower the rpm back to spec minimum. This apparently placated the EPA enough to allow importation. But, those carbs had no accelerator pump, and throttle response suffered and was only just acceptable. Some of the PD carbs had turn limiters on the idle mixture screws (IMS) as an “anti-tamper” compliance feature. 78 was the last import year for the 550. Note that the PD carbs of the 650 had accelerator pumps, and later switched to CV type carbs that no direct connection of slides to throttle control.
