The adjustment for the fuel stop is right in front of the FDC under a cover. The bolts are usually safety wired and sealed.
Remove lockwire, 2 bolts, and cover. You will now see a short piece of threaded rod sticking out, with 2 nuts on it.
One is a locknut, the other is the fuel stop adjustment nut. Loosen the locknut, but make sure you do not twist the threaded rod. This is important. Doing so would break it off.
You will need a thin 1/2 open end wrench to hold the adjusting nut while loosening the locknut with another wrench. If you dont't have a thin wrench, buy one or grind one down.
When the lock nut is loosened, increase fuel by turning the adjusting nut to the right. Do not turn it more than 1/3 of a turn without checking engine performance.
As little as a third of a turn will make a huge difference. By turning the fuel up too much, you risk melting your pistons.
It is also possible to increase max. fuel by opening the FDC cover and inserting washers or quarters, on top of the spring. This is the unofficial G.I. method. Soldiers used it because the FDC cover is not sealed, and this unauthorized modification cannot be detected by maintenance inspectors without opening the cover. It only works when the FDC is not bypassed. With the FDC bypassed, the piston is always in the lowest position, so you already have the maximum fuel that can be obtained using this method.