Berni,

My experience with an Aircoupe was enlightening.  I found the source of the problem to be a loop (imagine an inverted U) that trapped air, not fuel vapor, in a section of fuel line.  When the fuel valve was closed and the lines between the valve and carburetor were drained, an air bubble would trap in the high section of the loop when the valve was reopened.  On takeoff the high fuel flow would push some of the air to the Gascolator where it trapped at the screen.  Engines were quitting on takeoff and many times the bubble would get enough agitation on the hard landing to work though leaving no evidence in the glass bowl.  It was a faulty design later addressed by Service Letter.  A temporary fix was to force a high flow of fuel through the line to move the air bubble past the high part of the loop. 

Could something like this be happening on our planes?  Since we are running out of things to check, I am going to take a close look the next time I have the cowl off.

Steve   Legacy  IO-550
 
 Your comment regarding the air is interesting.  I don't believe we have any
air in any of the fuel lines but maybe I should look harder at this
possibility.
 
 Thanks for your input.
 
 Sent from my iPad
 Berni Breen
 Bbreen@cableone.net