The plugs will be black because you have a misfire, not necessarily because you have a mixture problem! When the fuel hits them and there is no combustion, they'll of course foul up whether or not the mixture is too rich, so don't go randomly playing with the mixture screw, otherwise you're likely to wind up with bigger problems

Have you got a good spark tester (i.e. not one with a light, but one that has an adjustable gap and has good visibility)? If so, see what the spark looks like and also run a compression test and then post results.

There are a bunch of common 16v misfire causes, but there's no point going through them without first checking the basics.