I have also encountered the problems that Michael is writing about above
Michael wrote:
Mind letting us know which aircraft types you’re referencing ?
Two PA31s. Each engine on each has a fuel selector with a cut-off in the middle, and a firewall cutoff, plus a crossfeed valve between the two fuel systems.
2 fuel selector cutoffs
2 firewall cutoffs
Crossfeed
= 5 per aircraft x 2 aircraft = 10, exactly as in the OP.
When learning to fly we always left the fuel tap/cock/shutoff open and stopped the engine using the mixture, and left the mixture fully lean. As a result I sometimes leave the fuel open on rented or group aircraft. Other than stopping the fuel at a different point along the fuel line, does it make any difference? I assume the shut-off valve is safer in the event of fire as it stops the fuel farther from the engine?
According to a Staggerwing owner, it has five fuel tanks with several selectors piggybacked one behind the other. Pretty confusing, and getting them in the wrong sequence or one failing is catastrophic.
I’ll test the fuel shut offs in future.
My fuel shut off gets tested at every annual, because you have to shut the fuel off when removing the fuel filter which is right under the fuel selector valve. If any fuel is leaking, the guy doing it will get it all over his face 
This is a great thread because these valves are great candidates for getting never looked at, due to the difficulty of dismantling them. The TB20 one is a particular bastard to do; I have a spare one which I got out of a crashed TB10 (may not be the right one for my plane but it looks identical). It would take all day to get it out and open it up.
I have written into my EASA Self Declared Maintenance Programme (SDMP) a function check and visual inspection for leaks on the stopcock in the SF260 on the basis that if I ever had a fire or engine failure I want to be sure it cuts off the fuel supply. A simple check that takes 5 minutes, the engine actually runs for 2 minutes before it stops due to fuel starvation.
Marchettiman wrote:
the engine actually runs for 2 minutes before it stops due to fuel starvation.
Presumably not if you increase power?
From my experience I can say that even the humble Rotax912 will start sputtering as soon as you advance the throttle with the shut-off valve closed.
If left in idle, on the other hand, it will run for what feels like forever on the fuel in the lines and the carburettors.
But I hardly doubt anyone will be able to taxi to the runway, do a run-up and depart on that amount of fuel. And this is a very economical engine.
Btw- I learned this the way Peter mentioned, after doing the 100h-inspection on the aircraft during which the fuel filter is checked and cleaned.
Found out while I was trying to taxi to the run-up area to do the test run prior release to service.
Wasn’t in “go flying mode” so didn’t look at the checklist.
Lesson learned!
I’ve checked this with the Jodel DR1050 and think that on a strip, where you start, do checks, enter runway and take-off, you could get enough speed to be unable to stop, and probably get airborne into a strong wind, before the engine starts to sound different.
I’m wary of Pilots Notes which say to change tanks before run-up and take-off. I prefer to change tanks soon after take-off, at a safe height.
I have three times had the engine start to die, switched back, and returned.
Once fuel blockage, once fuel leak, once air leak into mechanical pump. All affecting the rear tank, which is not used for take-off.
The DR.250 I sometimes fly has an annoying problem in that the low fuel pressure light comes on when using the rear tank. Turning the selector slightly past ‘rear’ toward ‘right’ and back again seems to cure it. The whole system has been taken apart but to no avail: it’s always the back tank. Maybe a Jodel/Robin thing?
Maybe a dodgy pressure switch? Isn’t the switch downstream of the electric fuel pump?
