Dry, black, carbon soot on plugs says that you have ignition but there is too much fuel for the oxygen present in the chambers during combustion.
The fuel is metered by the slow jet (and it's fine tune air bleed screw), throttle valve, and main jet, that each provide dominant mixture, depending on where you position the twist grip and what load is placed on the engine.
Some people drill their jets to a larger size instead of replacing them. This makes the numbers on them meaningless. Engine runs rich.
Throttle valve needles and the throttle valve orifice can vibrate against each other and wear, increasing the metered fuel quantity. Replacing half of the pair only fixes the problem half way. Engine runs rich.
The engine is supposed to run on the rich side at idle settings. This provides good throttle response under loaded windup, since you have no accelerator pump to richen the mix when the throttle is cracked open. Prolonged idling can blacken spark plugs.
Small adjustments in mixture can be achieved with changing the float height or fuel level in the carbs. This usually has some effect on all three metering devices.
A diagnostic tool is to temporarily run the bike without the air filter to eliminate its mild choking effect on the carbs. If the mixture the throttle setting you are adjusting improves, then lean the mixture for that setting. Conversely, if adding partial choke improves that throttle setting, that mixture device need to be enriched.
Your bike was modified before you bought it. And, it sounds like someone sold you their headache. You will have to become smarter than they were to tune for the modifications made. Or, return the bike to stock Honda configuration where all factory settings are known and apply.
The stock configuration is the only magic bullet I know of, since they worked 1000's of manhours to get all the adjustments and settings correct for that configuration. Changing components, means you get to repeat some of those manhours to compensate for those changes. If you're smart and methodical, you spend little time. If you just keep swapping pieces, luck determines how long the process takes.
You are going to need a Dyno or a test track, to load the engine at various throttle settings. And an exhaust gas analyzer, or spark plug analysis with tell you when you've got the mixture right. This process can be fun in and of itself. Or, it can be drudgery detracting from other things you'd rather do. It's what you make of it.
Sorry for the stump speech.
Cheers,
