Sit at idle and turn the fuel OFF. See how long
it takes the engine to quit? Never change the fuel after doing
the run up.

I replaced the seals in the 172H fuel selector (the one noted above),
after finding that it wouldn't fully shut off. One o-ring on one side
had poked out of its groove such that it would maintained idle etc
indefinitely, but I'm certain it couldn't have supplied takeoff power.


I agree never change the fuel selector after runup but think under some
conditions there would be enough in the bowl given a leaky fuel valve
to do a runup. The genuine off function isn't a problem, but the leaky
off condition is really dangerous. That's why in my present 172M I'm a
little hesitant on general principle, to always turn the fuel off, just
to get an easier start.