How many miles on the new exhaust system? Have you driven it at long (20-30 miles) intervals? Some new systems have coating on the pipes which burn off rather quickly. If you drive it and heat it a few times you should be O>K> I mean you shouldn't have any new exhaust burn off as a problem.
I also think that any black carbon gas smell from the exhaust is likely to be with the tune-up/injectors. Are they original bosch injectors with new boots and seals/rings?
The other option is to have the exhaust analyzed. There should be a state inspection station or private shop that could shed some light on this for you. Good luck.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'