Well, you're "right up against it" with those PD46c series carbs: they are touchy bas#$@ds to set right enough to run with the 550 and modern fuels. This has long been the reason the 1978 550 bikes are often found with low miles, mysteriously parked: I still remember when we had both MTBE and ethanol in the fuels here at the same time: there was a guy with a nice '78 near here who got so tired of tinkering with his carbs that he drained the tank and put a For Sale sign on the bike in his front yard. The empty tank was so that a prospective buyer couldn't try to start it up, because it would just rev high when cold, then die when warmed up.
One thing you might try with the 550 (it works very well with the 750K models) is to add some oil to the gas. I use about 2 ounces of oil at every fillup: it helps neutralize the problems caused by the gas itself, letting you troubleshoot things in a more sane way.
For the too-rich 750F: there's several things that cause this issue. In order that I see most often around here are:
1. The spark advancer is advancing too soon. This makes it spit back toward the carbs (this is also big on the 550) and causes the airspeed thru the carbs to run slower than the engine ingests air, and since the carbs all run richer-per-air-volume as the air slows down, the result is a rich, dark mix in the chamber - and fouled sparkplugs, by the box. To fix this: start by removing at least 1/2 turn from both springs on the spark advancer to slow it back down: its springs are now heat-softened and let the weights advance too soon. Try for full advance at 3000 RPM, not the original 2200 RPM in the manuals: this matches today's fuel burn rates better.
2. Make sure the rubber hoses between carbs and head are still supple. If not, they are very, very difficult to make seal. When they leak vacuum, this creates the very same scenario described above, resulting in rich, dark sparkplugs. In th e750 this is caused by hard hoses and/or stretched-out band clamps on those hoses: in the 550 it is caused more often by the O-rings in those cast intake manifolds where they contact the head: they are old and hard, now, and leak vacuum. They are the same size O-rings as in the valve caps (30.8x3.2mm).
3. Over-advanced (at idle) spark timing. In the 750 this is more often seen with the Dyna electronic units (just before they die) when their 'trigger pickups' are starting to give up: they will show, under a timing light, that they are firing too late until about 1500 RPM, then they jump forward to 50% of the actual advance, then follow the advancer above 2000 RPM to whatever angle it is making at the time.
Also; don't use premium gas, especially if you're getting dark plugs. Our Regular grade gas now burns slow as (or slower than) the Premium grades of the 1970s, so this messes with the engine's low-speed response in a dozen ways. Adding a bit of oil to the gas also helps tame this burn-rate problem.
