I am trying to locate a air/vacuum leak. The vehicle runs rich (smells like it anyways) very fumey. I hear a ssswwwwwsssshhhh at the point of shift during acceleration. Once the tranny shifts into a higher gear the noise goes away or it is not as loud. I have replaced all vacuum lines on top of the motor and sprayed at least 2 cans of carb cleaner trying to find the leak. So, my question is, does the fuel return line play a role in this? Will it affect the fuel mixture and or fuel delivery?