My 1978 GMC Jimmy w/ a 350/t-350/np-203 Has developed a miss.It has a slight miss at idle when warmed up,and when accelerating it stumbles/pops/misses or coughs back through the carb. The carb is a remanufactured quadrajet Which is about 4 years old and has about 30000 miles on it. The motor was rebuilt 2 years ago.I have checked for vacuum leaks and can find none. However the carb seems to have a louder than normal hiss?Help WTF Is wrong? I sospect the carb is shot or ? Any help is greatly appreciated!Could the power valve be blown. I guess it's time for a Jiffy carb kit ay.