well, it's mechanical. providing the pump isn't starving (filters-intank and just off tank on rt rail), timing chain is toast or the pump is.
a full toyota factory service manual for your truck can be dl'd here,
http://www.ncttora.com/fsm/87-4Runne...4runner_88.pdf
it covers your pu.
trouble shooting the carbs fuel system starts on pg 294. an illustration for location of related components is on pg 321.
it's free.
owner 93 & 88 toy pu.