Years ago, I desmogged my 2F, so I was venting the OEM charcoal cannister to atmosphere per Jim C's diagram at the time. Also my OEM fuel tank's bottom was rusted and leaked. So I replaced it with an OEM unit and changed out all the rubber hoses (filler & vapor). That cured the fuel smells for a bit. Now, a few years ago, and few engine up-grades later, I started to smell fuel while driving again. I never could find the cause till I removed the evap cover, wiggled the lines and found I had a broken nipple on the plastic evap. It was at this time the OEM cannister was checked and I decided to remove it. So, I made a few adjustments, installed a four-way connecter in lieu of the evaporator, ran the vent hose down, out the frame and added a 12" long 1/2" hose whip filled with the carbon. This solved my fuel smell....
Now, the reason for my new vapor cannister, ran across a U-tube vid showing this modification on a guys home built roadster a few weeks ago. So I gave it a go and upgraded to this cannister unit. Went from about 2 oz of carbon in the whip to 12 oz in the cannister, which is easily replaced when needed.