With, or without, the, "Secret Reserve?"
The reserve fuel amount (volume, not pressure) is determined by the petcock standpipe height. Exchanging OEM vacuum petcock to a manual version may reduce the effective available reserve volume, because of shorter standpipe height in the alternate manual component.
(A friend of mine found out this is so!
Now, a weird saga from KLRiana: A sincere, helpfully-intended guy discovered a technique for converting the OEM petcock from vacuum-actuated to manually-actuated; wisdom he freely shared . . . may have even made some kits to sell or give away. He also promulgated a misapprehension, cutting petcock standpipes affected fuel pressure! Ain't so. Static fuel pressure to carburetor remains solely a function of the "head," the vertical elevation distance between the carburetor and the fuel level in the tank. Standpipe height is of no consequence to fuel pressure in a gravity-fed fuel system such as the KLRs.
Yet . . . some beneficiaries of his vacuum-to-manual activation petcock modification dutifully followed his instructions regarding standpipe height, in hapless expectation of increasing, "fuel pressure." Some established physical principles require revision for successful realization of this expectation.