The 'weak springs' situation is caused by heat-annealing of the springs on the spark advancer plate. Almost 100% of these aged bikes are affected by it: just cut off 1/2 turn of the spring(s) on the advancer and re-bend the cut end out to a half-moon to grab the weight again. I do it to almost 100% of the engines I rebuild nowadays, as they are all in this situation.
The 'cutout' of a cylinder or two is also possibly caused by erratic air pressure in the float bowls of the carbs, given the configuration of your bike. There are (were?) 2 little vent hoses from small elbows on the center 2 carbs that went to a quiet, still-air zone behind the original airbox: these vent hoses are cross-connected to the #1 and #4 carbs also by little vent hoses connecting them, at the same height location on the carbs. This is a very important item: the still, quiet air that originally fed these hoses caused the high pressure in the float bowls that pushes the fuel up the jets at throttle speeds over 1/4 open. If this whole subsystem is gone, then it is almost impossible to get the fuel pushed up from the float bowls into the throats of the carbs. Keep in mind: these are tiny engines, just 4 of them coupled to one crank, each one about the size of a lawn mower engine. So, think of them in that way when debugging stuff.
When one of the pair of carbs that is fed by one of these vents hoses loses pressure, the next cylinder in the firing order ends up with a heavier load. For example, if #2 carb's vent hose is missing, it will fire lean because the fuel can't push up its emulsifier tube: this makes #1 then suddenly experience a slightly heavier load, despite that its throttle setting is the same as for #2. This makes the whole engine uneven then, because this ripple slows the crank slightly, making #3 the next to [now] have a too-rich mix because it took in fuel at a slower rate than its throttle setting. After enough of this happens, the plugs get dark and the engine gets 'wet'. It all adds up to poor running.
When the OEM coils (now with 50 years of aging on 10-years wire insulation) start to fail, they usually fail when hot and at higher RPM. They misfire when hot, run OK (not great) when cold: that's the symptom of the coil past its age. Check out the ones at PartsNmore in their CB750K section (#17-6823) for a decent low-cost replacement that will pretty much drop right in (if you don't mind mismatched wire colors).
