I've eliminated vacuum petcock on two old carburetor bikes in the past, one of them if I left petcock on it wouldn't get pass the carb, the other one however would get into cyl/motor which sucksss happened more than once before I learned ..LOL really bad if I parked it with a full tank.
Only thing I can figure is condition of needle/seat in float bowl or ??? I did find a old off the shelf replacement whatta you call new old stock, and it failed after no time.
It really wasn't a problem remembering to shut it off/on, cept the time I let a ole friend ride one and forgot to tell him about it and few blocks away it quit running..he knew what was up and continued onward !!
Old Biker told me once..Its doesn't matter how many brain cells you kill, but how many you had to begin with ?