From what I understand, modern petrol does not give the same "discolouration" as that used in years past. Again, I have been told the only way to check the mixture "correctly" is by using a lambda sensor. One other issue with modern plugs is the ceramic is not glazed. This can be an issue with carburettor engines as the can flood the plug with fuel and cause it to misfire. Better with modern plugs to put them into a "warm" engine as this will enable the ceramic to get coated with carbon before being next flood with petrol on a full choke. Problems I had usually resulted in misfire.
ATB
