the three slots in needle are for changing the fuel/air mix
the top slot (needle highest in the jet) = a richer mix, black plug.
the middle slot (needle centred in jet) = std position, good plug (hopfully
the bottom slot (needle lowest in jet)=a weaker mix, glazed/white dotty plug.
The needle controls the mix of fuel/air around 1/4-3/4ish throttle position.
Symptoms of a weak mix on the needle are spitting PoPing from the carb
usually when opening the throttle, it might settle down (slightly weak) or do it all the time (very weak)
If not corrected a weak mix all jets included will overheat the engine and devour the piston crowns.
Precursors to piston failure would be a glazed white plug, overheating, pinking, loss of power
A rich mix on the other hand is not as bad, the biggest danger is petrol flush.
Symptoms of a rich mix would be a black sooty plug, lack of power,
poor throttle response it may give signs of a slight miss fire.
A good starter when cold hardly needs choke and a bad starter when hot, opening the throttle can help make it start.
Clean your plugs and if non of the major symptoms
above are apparent, go for a ride
do a few normal miles, whip your plugs out and check the colour,
do a few more miles at a set throttle opening and check the colour.
I have noticed with the modern fuels plugs tend to look a bit whiter
than they used to,
what you don't want is a glazed shiny white electrode usually accompanied with grey dots
(probably specks of piston.)
With standard jets and factory settings on a standard motor
most of the above is unlikely and only fine tuning to suit conditions will be required