I can't get the Thermoquad on my '73 400 to quit running too rich. I put a carb kit in it making sure to install the primary well seals correctly, but this only helped a little and I still have black exhaust coming out of the tailpipes - I have the idle mixture screws turned all the way in, too. What are the next things to look at being the culprit? The only thing I can think of is it might have a cracked fuel bowl since its made out of plastic and over 46 years old (so it's got to be getting brittle by now). I'm wondering if this carb is even worth investing any more money and time in and whether I would be better off getting a replacement one.