I'm currently reading the book Through Gates of Splendor, and at one point (pages 61-63 in the first edition, about halfway through Chapter 5), it describes a system invented by missionary pilot Nate Saint that provided a backup to the main fuel system in the Piper PA-14 Family Cruiser that he was using for missions flights. Because the book is not primarily about aviation, it doesn't go into a lot of detail on how the system works. The system was designed to protect against problems "such as clogged vents and broken lines."
Is there any more information available on it?
It apparently bypassed the entire fuel system, including the carburetor, and used fuel from an extra tank strapped under the left wing and made more aerodynamic with a balsa-wood fairing. Throttle was controlled separately, not by the plane's normal control, but he did add a control rod to the instrument panel; it appears that the rod simply opened and closed (or adjusted to partially-open settings) a valve. The system was able to function without a carburetor because the engine was still windmilling when it was started(?). (I didn't know a non-fuel-injected engine could run without a carburetor, but apparently it is possible if the engine is already spinning at a high enough speed?)
He flew to 2,000 feet on the normal system, then "pulled the mixture control to idle-cut-off" (I think that means he turned off all fuel supply to the engine?) and turned the handle he had added for the backup fuel valve. The engine "wound back up to a smooth full-power"; he flew with this system for twenty minutes. He said that it worked just as well as the primary fuel system.
The system weighed four pounds including the (empty) tank, and must have been quite simple because he built it with only the extra parts he had access to.
This system was installed in at least two other planes, because it says that all Mission Aviation Fellowship "now" use the system, and when this book was written, the original plane had already been destroyed. (I don't know if they still use the system; the book was published in 1957 and my copy appears to be the first edition.)