Pumping bubbles

[Doug Rasmussen <76075.2276@domain.elided>]

John H wrote this in a response to Michael's engine problems:
>I would watch the stream from the fuel line to see if air 
was being pumped in addition to the gasoline. <

John H,

   Funny you should mention pumping air.  Once in a while, I notice my
clear plastic in-line gas filter on my 304 looks like an air/gas mixture is
being pumped.  It's kind of pulses of air bubbles.  Of course, the only
time I can observe this is when it's idling.  Most of the time, it's just
clear with gas, no bubbles.

  I'm not having any problems when this occurs.  I'm only wondering what's
causing this.  My gas tank isn't low.  And plenty of gas is getting to the
carb for acceleration.   No external fuel pump leaks, etc.  

  My concern is this is a subtle hint of a pump getting ready to fail.   

Doug