How long is a "while"? Float valve is a metering device, it should not be looked on as a shutoff mechanism. I reckon if the car idles Ok for 30 minutes without flooding occuring and can be left switched off with the tap open for 15 minutes without flooding then it's OK. Stopped for anything longer than 10 minutes and you should always turn the tap off.I have installed a Tilitson on my Model A. It runs fine but leaks fuel after sitting for a while.
Bob