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A bad selector valve should be stuck in the left or right tank position if it is bad. So there may be a problem with your fuel system to include the carb.
Check fuel filters, see if there is fuel flow from the pump, check the selector switch and selector valve for power - if they both work you should hear the selector valve when you depress the switch for 2-3 seconds and release, then if problem persist check out the switch and then you can remove the selector valve bench check it and ensure it works and fuel pathways in the valve are clear (they can get clogged up), and inspect the fuel sending units and lines/hoses. Could have kinks in lines and hoses.
The tanks and system can get gummed up and rust is possible, especially if the truck has been sitting for awhile.
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