So on a clients 1996 Heritage we put True Dual Exhaust System and a Wyatt Gatling 29" Muffler Set and after cleaning the carb and adjusting the mixture screw I notice that the spark plugs are black (no oil, just dry black) and a bit of black smoke comes out of the exhaust. It has a #42 slow jet and I'm wondering if I have to get a #40 for it? So my question is, would an exhaust setup like we put in actually block more the gasses than a regular exhaust that would need to lower the slow jet?

 

define cleaning the carb
just a soak or a strip and soak.
also, is the float height correct
on some carbs, you can install the main holder upside down and this screws with the immersion well so it will burn like logs instead of twigs.
i would not think you need a lower slow since hd was pretty lean to begin with and a lot use a 45.

 

It was a complete tear down of the carb and I put a new rebuild kit... I will check the needle jet to see if it's upside down (this is what you where talking about yes?).

 

Yes, number 11. There are a posts with pics showing you what correct and incorrect positions look like so no need to take the carb apart to determine. Just search the carb section and the pics will eventually show up.