Originally Posted by Lars

The check valve inlet filter is actually a roll-over fuel spillage prevention device. GM found that in the case of a vehicle rolling over, the fuel tank ends up being higher than the carb. Depending on the attitude of the tank and the fuel level in the tank, it is possible for fuel to flow from the tank, through the lines, through the pump, and right through the carb. You can imagine the possible consequences of being trapped in a rolled-over vehicle with fuel pouring out of the carb and onto the ground… So GM installed a filter in the carb with a spring-loaded check ball. This check ball requires a certain pressure to unseat and allow fuel into the carb – easy for a functional fuel pump to accomplish, but the pressure of gravity-fed fuel from a roll-over cannot unseat the valve and allow fuel flow. Roll-over problem solved.

But the roll-over valves also assisted in solving another problem, as you can now see. The return-style fuel pumps that came into use at the same time as the roll-over issue could not positively prevent the fuel from draining back from the carb due to the internal bleed orifice for the return fuel. The roll-over valves seemed to solve the problem, even though they were not designed to do so.

Yet problems persisted, because many people removed the “restrictive” check valve filters and installed the early-style filters without the valve. Also, if the filter pre-load spring is missing or weak, the check valve filter will not seal and prevent reverse fuel flow.

Originally Posted by 8T8T2

Removed fuel filter and it was dry. At first I thought filter had been installed backwards but it wasn't. Found the little check valve -I'm guessing thats its purpose- inside the filter on the inlet end was stuck shut. After a push with a small screw driver it popped loose but didn't move much. Checked the old filter and got very little movement from it also. Seems like a hell of a fuel restriction.