Honda-SOHC

Hi Need advice
My cb500k
All this while was running ok
No issue plugs colour was good
But recently all the 4 plugs have black dry carbon black

I did check n replace ignition coil
Spark plugs caps and plugs
Did check ohm resistance using multimeter
Bike using CDI
Did do Static Timing to F mark
Did valve clearance and cam chain adjustment

Need advice if its due to main jet can it been suddenly ?
I mean all this while was good with same jet
Carb fuel level was good too

Thanks