On my 67 318 the heat riser is missing the spring and weight. I don't know if it's stuck shut but when it starts it misses like a bad valve. After it warms up it's a lot better but generally runs like crap. I can't set the mixture on the carb (won't pull it down when I screw them in or kill it). It's a new carb and does the same thing as the old one. Any ideas? Thanks