Well, basically.... the troubleshooting begins....lol..

Just one plug fouling like that is usually caused by that plug not firing consistently or bad compression in that cylinder. So...it would most likely be caused by a bad plug or a bad plug/coil wire or bad connection or something. But it could also be something carb related that's making it run richer in that carb than the others (like maybe float level or something).

 

So,went through the whole process again this morning.................
Aha! No fouled plugs.................................

Cleaned all the plugs refitted them,started Bike....Still a Duke 999.
Walked 5 kms to the Hardware to get a big enough Fuel can to empty the Tank into.
Drained the Float bowls while i was at it.
The Fuel wont drain unless the Bike is Running or trying to run......lol,but you all know that anyhow,dont ya?
Went & filled her up,went to work,better response now on the gas,maybe the float bowls play a part.
Im happy & i have enough Lawn mower fuel for 12 months.

Heres that plug.

 

How could bad gas affect only one plug? Makes no sense. We aren't at the bottom of this yet.

So, it fouls if it isn't firing or if it isn't firing fully. Right?

So, as Trout says, not enough spark or too much fuel. Right?

I don't remember hearing anything about a distributor so I assume these have an electronic ignition? Do they even have a coil?

So it can be in the source of the ignition ("chip") or the wire from the chip? Check the wire connections/clean them up/rough them up? Or swap wires to see if the wire is bad? So if you have the right amount of spark, it has to be too much fuel or oil intrusion from bad rings?

Am I warm?

 

As far as im aware Joe,Black or fouled plugs can be derived from the following:
1.Over rich fuel/air mixture
2.Blocked air filter
3.Choke faulty..........

I put it aside with the fan switch,reg/rec ,headlight globe all the stuff downgraded to "When i get around to doing it"

 

Glad to be of service. WTF do I know? I don't know sheit about electricity or ignitions. Frankly, I'm surprised an electronic ignition needs an old low tech coil. But that just shows what I don't know.

Black plug (I used to foul them all the time in my two-strokes) means the stuff ain't burning. Just going through the list of what could cause that. Kind of a thought experiment. (Einstein used them so you can KMA - ) Anyway, seems like a long list potentially.

I understand a choke being stuck closed. (I presume each carb has one and they all get yoinked together with some cable/bar arrangement?)

And clogged air filter gives no air. (but wouldn't that affect them all?)

And rich fuel/air which means those screws are wrong on just the one carb.

Don't really mean to eat your time, but trying to learn here (and hopefully some dumb a$$ like me who isn't willing to publicly humiliate themselves might learn something with me.) In my biz, people make a lot of mistakes by jumping to conclusions and not starting out with the fundamentals. It's kind of irritating to those with great familiarity, I know. Sorry.

 

Hey Joe,motor car-bike -skills/mechanic i aint either,no ****ing way.
Theres 1000's of prognossis for this fault,machine/circumstance.
What the hell,the bike is Running,im happy.Now if the firing order started with cylinder # 3.........................
Joking man,i still blame the fuel!!!!!

 

Yup,quite warm mate,good on the Bike,Thai takeaway...............

Not doing nothin to the Bike,its runnin,so best not to **** with it..

 

I agree that if only one plug is fouled, something is wrong with that carb. The main jet may not be fully down at idle. This could be a sticking slide. Either pull the slide and spray carb cleaner on the slide wall or run a tank through the bike with injection/carb cleaner. Hopefully that will remove any buildup in the carb and you won't have to pull it apart.

An other thing that could be happening is that the enrichner is not going back in fully for that carb.

Of corse, if that carb is out of ballance it could be to far open, but I doubt that.

I would put enjection cleaner in it and ride it at highway speeds to clean it and then check the plug.