Going through a carb rebuild and was wondering what the factory used for the carb filter. I know the correct filter was the paper filer, but they made one with and without a built in check valve. One of the carb tuning papers actually suggests to use a filter with a check valve to keep the gas from running back into the tank after the car sits for a few days. I'm sure most of you experience hard starting after the car sits for a few days. The fuel pump has a check valve as well, but it may not be enough. Just wondering if the GM engineers covered that back then. The check valve in the fuel filter is suppose to keep fuel from running through the lines, keep it in the carb for easier starting. Napa part Fl 3051.