Ichi,
The PD carbed Cb550s are more frugal with fuel mixtures than the CB550's with the earlier carb style. I do have to operate with more choke and for a bit longer on my 78. The 78 also gets better gas mileage than the 550s with earlier style carbs. I note that the 77 has #38 Pilots, while the 78 has #42. I haven't run a 77 through a warm up cycle, personally. But, I can imagine it being more lean and needing more choke to coax more fuel out of the smaller pilot jet.
If you had the inclination, you could up the pilot jets to #42. And that should help with the cold throttle response. Alternately, you could try opening the pilot screw another turn or 1/2 turn outward, just to test the lean idle mix assumption. It should improve throttle response, especially during warm up.
The D7's really are better for the CB550 unless you plan to do flat out runs in the desert high heat.
Another item of note is the fast idle cam adjustment. It is proportional to the amount of choke applied. Full choke works out to about 3500-4000 shortly after it warms up. This can be adjusted so that less throttle up is achieved at full choke. This allows more driveability at partial choke without excessive throttle racing.
I don't like to race a cold engine, so it is natural for me to push the knob inwards to reduce the idle speed. However, this also reduces the amount of choke applied and leans the mixture for iffy operation when cold. The older carbs have the choke plates operate independent of throttle control. So, it is easy to find a choke setting that yields descent throttle up response, and the idle speed controlled by the twist grip as well. An equivalent arrangement can be made to the PD carbs choke/idle speed adjustment to achieve similar enrichment boost. It's a bit fiddly, because you get only one shot at a totally cold engine. So, it may take several tweaks to get the balance right. But, once done, it works out quite conveniently.
