I have a vague recollection from many years ago an Engineer telling me the Cessna 172 fuel selector was not a left / right selection but was just a bias towards the selected tank. I.e. If you selected left tank it would draw mainly from the left tank but would still draw a percentage from the right tank and vice versa.
Can anyone shed light on this?
The gravity feed system of C172 is always good for a lengthy tell-tale campfire ...
The C172 has an overhead tube connection between the wing tanks. In the beginning left wing tank had a closed fuel cap and the overspill tube plus open vented fuel cap on right tank. The idea was to vent the left wing tank via overhead join tube and right vented cap. Didn't work out well ;-). They later switched to two vented fuel caps, which did not compensate for unequal tank usage ;-). The overhead join vent tube between the tanks is not exactly on top of the tank, so you always have some partly cross feed (which is the reason why "use only one tank and then you have the same time on the other, is potentially dangerous" = never do that in a C172). On some C172s it can be several gallons cross feed via the overhead join! The changed the tubing later in production and, if memory servers correctly, now now have a curved overhead join tube, which extends above top level of the tanks in the middle, so preventing cross feed.
As there are many different tries of Cessna to work on the tank feed problem, I always advice to use the original check list for that specific built. A C172E
is different to a C172N ...