OK... now we are getting somewhere... the plugs are fouling? You did the coil mod? First, most plug fouling due to carbs is cause by the pilot circuit or mid-range. The main jets meter gas ONLY when the bike has a wide open throttle so I would guess you are not running around on wide open throttle when the plugs foul. Since the puny #15 pilot jets (stock) are not likely to be the culprit, and you can't get any smaller pilots, I would adjust the idle mixture for the highest idle speed using either the air jets or pilot mixture screw (on bowl bottom). I don't know which your bike has. Once you have the highest idle, turn the idle speed down using the idle knob. If that doesn't fix the problem, then you have narrowed it down to the aftermarket jet needles.
The Y63 jet needles are replacement for the 5DL31 jet needles and you didn't hve 5DL31 jet needles. You had 5CL10 or 5CN16 or something like that. These were matched to your needle jet which is the brass doo-hickey sticking out of the bottom of your carb's venturi. If you have done the coil mod, I would say that the Y63 jet needles are your problem... You or someone swapped in the carb kit jet needles and these are thinner and longer than the ones they replaced.
To try and fix the problem using those aftermarket jet needles, try lowering them two clips from where they are now... IF you clipped them at the fourth slot, clip them at the #2 slot counting clip positions from the top. This will LOWER those jet needles and a wider spot will occupy the needle jet at almost all throttle positions, letting less gas through. You will need to resync the carbs again when done.