Never mind the exhaust, the brownish grey deposits we used to look ofr were caused by the lead in fuel. Look at the plugs, if they are sooty then you need to worry, if they are brown then all is well.
IIRDC you can adjust the mixture by adjusting the VAF sensor (the flapper) somehow on early EFi and similarly on the MAF sensor of the hot-wire. If it's running cats then leave well alone.