getting such a red brown plug with heat range of ND 16 means that either your engine has lost compression or you are feeding it too much fuel - remember that if you add too much fuel - your oil intake ratio goes off. Resulting in piston scuffing etc.
Put a BP7HS - NGK plug in it, and adjust your carb to factory specs, then work with port timing and squish clearance to get the maximum power out of your machine.
This is how you should adjust your carburettor. Install a kill switch on the right handle assembly.
mark the throttle in 4 marks -
install new or cleaned plug and ride the machine at 1/4 throttle and kill the engine from switch when the engine is under load and immediately pull the clutch in, stop and remove plug - inspect - if rich, your idle jet and idle mixture needs adjustment
ride at 1/2 to 3/4 throttle and kill engine as described above - this time you need to adjust the needle or holder
ride at 3/4 to 1/1 throttle - and kill engine - inspect plug, this time you need to work with the main jet size.
If you let the machine coast to a stop with engine running - your test is useless because if you were trying to adjust a high end lean condition and coasted to a stop with engine running and your idle and 1/4 throttle were rich or near perfect - you will see a perfect plug and will mask your approach.
Before all this you need to make sure that
your carburettor is clean from everywhere, including all the tiny air passages
your carburettor float height is correct
there are no air leaks behind the carburettor.
Your ignition system is satisfactory.