Grrr... it now looks like it is the intake boot, not the vacuum cap, that is leaking. She will start right up and run perfect - for about a minute until warm. Then the hunting idle speed begins, followed by intermittent pops and small backfires. It all appears to be on the left hand carb, as I can feel it kick back each time - but not on the right side carb. I was watching it closely when it started doing its sputtering stuff, and now I can visibly see a slight 'puff' of fuel vapor squirt out the bottom of the intake boot on the cylinder head side every time it backfired. That is where the fuel came from that I thought was escaping from the vacuum cap.
