talkturn :
To my knowledge on the R22, there's no breather pipes because the tanks caps are vented. The fuel is gravity fed as well, so that's out. The only thing I've heard of is a problem with the connecting tube from the aux tank which was a factor in a fuel starvation incident some time ago, but that's about it.
Otherwise the only fuel starvation incidents are plain old none left in the tanks, or ( as I once heard ) a passenger who used the fuel cock as a convenient place to hang his coat, which turned it off !!