I've seen both the too-lean and too-rich aftermarket (needles + emulsifiers) in the 500/550 situation: I rarely get to work on the CB400 around here (because all the ones I know of work just fine, still - good bikes!) to discern whether their aftermarket needles and emulsifiers are rich/lean, or which way.
For example, just for reference:
in the 750 PD carbs, using the K&L or the Keyster jets will make them about 3%-5% leaner than the number stamped on their jets, and using their emulsifiers with their needles will make it even (much) leaner than that as their needles are much thicker overall (and the emulsifier holes are correspondingly larger). The struggle here is: it actually gets so lean that the engine misfires every few revolutions above 2500 RPM, and the unburned fuel from that misfire builds up in the cylinder and header, then fires at less-than-cadence speed, making that burn event very rich. Then, because it is SO lean, the engine balks when warm and black soot builds up everywhere. This baffled me for a long time until I figured it out by accident: I installed mainjets that were 25%+ richer than they should be (#140 instead of #105), and it worked perfectly. That was in 2010, I think, on a 750K7 I rebuilt to new, from junk. Those were all Keyster parts, in PD41 carbs. Because of the 3 other cylinders being so eager to turn over the ones that are not firing so well, this becomes more than confusing...
In the earlier CB750 carbs, the K&L/Keyster emulsifier just has incorrect aeration hole sizes, easily fixed up. Their needles are also thicker, and their supplied emulsifier jet (in the throat of the carb) matches their needle fairly closely in ratio the the Keihin version, over the travel distance. BUT - the needle jet is a separate part, and if used with the Keihin needle, its hole is much larger and uncontrolled richness results. The fuel is simply being sucked past the needle at much higher rate than the slide's opening, and the sparkplugs foul in about 5-10 miles from a rouhgly 6:1 A/F ratio (normal is about 12:1 here).
So...whenever non-Keihin brass is involved, I go straight to that as a known issue from using those parts elsewhere, although the actual problem may be different in the 400F: there are also 2 different 400F carb setups: the 1975 version and the later one. They are different: the later version is more 'touchy' than the earlier one because it had emissions rules to follow.
I might suggest trying to install all Keihin brass if you can find it all?
Another tip:
Make sure of the spark timing. If the spark is advanced too much, it makes the engine spit back at the carbs at lower speeds (less than 4K RPM). This slows down the airspeed thru the carbs, making them run richer than the engine speed might otherwise require. I test for this by simply retarding the timing by turn the points plate to the right a little bit, so that the points open in between the "T" and "F" marks. If the carbs' situation seems to improve a little, then the spark advancer's springs have become too soft from the years of heat-annealing, and they should be shortened a little bit to slow down the spark curve. But, this presumes FIRST that the carb-mixing situation has been set up properly: it won't cure improper parts, nor work if the mix is just too rich. In this situation, it cures things like sparkplug fouling in 100 miles, and the like.
