Why do you want to add a fuel shut off? Is it leaking through the carburetor and flooding out the carburetor or dripping out or getting your tank low or getting fuel into your oil?
If so it needs a repair to fix that problem as the fuel shut off is just a Band-Aid for this but not really a good one.
If these things aren't happening then you don't need a fuel shut off for any reason.
If you're following the old school mentality of running it dry or you're trying to turn the fuel line off every time you shut it down to drain the bowl for some sort of benefit for starting or staying cleaner etc then that is also not good practice.
I know people have said it for years and some manuals even talk about running machines dry and stuff for what they call long-term storage but they never say how many weeks or months or years long-term storage is.
In climates where there is an off-season, running one dry or what people call dry, because it's never really dry, is better than doing absolutely nothing and just walking away from them after the last mow but it's certainly not the best thing you can do.
The best thing you can do is to make sure before you mow with it for the last time that you go by fresh fuel and add 1 oz of fuel stabilizer like sta-bil and your gas can for each gallon of fuel you're going to buy. This is a stronger long-term storage dose and then fill your tank on your mower either all the way full or with a substantial amount of fuel because you want that fresh stabilized fuel circulating and going everywhere and fully saturating and being the largest volume of the fuel in there after it mixes with the old fuel which may have not had stabilizer.
So after you mow for the last time this has done all that.
Then, you should pour the rest of that fresh gas in there and top off your tank because it's far better to store them with a full tank than an empty tank because of condensation and moisture but again the climates have a lot to do with this.
This is far better than trying to run one dry or leaving it mostly empty because when you run one dry you basically only get it half dry..
The ball has about a quarter to a half of its level of fuel and the fuel line ends up drying out completely as does the needle and seat which has rubber somewhere involved.
These rubber pieces will start to dry rot and harden more quickly than if you left them saturated and fresh stabilized fuel.
You also must remember that the small amount of fuel in the bowl will evaporate and leave something behind and with recent gas it's quite a bit of bad something.
It simply safer to let the bowl stay full.
It's never a bad idea to have a fuel shut off for the off season so just in case it does decide to leak or something you won't lose all your fuel and make a mess but it shouldn't be done as a replacement for storing them with fresh stabilized fuel and a full tank.
