I started smelling fuel during power back after takeoff or when I opened the canopy while taxiing on the ground. In flight, opening the heat vent or the side vents quickly cleared the smell. The usual sources, such as carberator bowl gasket or spitting fuel out the carburetor vent line were clean and dry.
Finally checked the fuel tank vent line aft of the bulkhead. Found where the fuel had been collecting. Notice the tightness of the small vent line. It had shrunk a substantial amount as slack in the line was gone.
At the top, the shrinkage had pulled the tee fitting down a substantial amount from near vertical and the flexible vent line easily pulled from the tee. The upper portion was hard and stiff. Overfilling the tank or taking off with a full tank would cause fuel to flow through the tee as it should. However, fuel would also flow into the third leg of the tee and leak into the aft portion of the fuselage.
It appears that the pressure change in the cockpit from pulling power after a climb or raising the canopy on the ground would pull the fumes from the tailcone and into the cockpit. Opening the vents while in flight would increase the cockpit pressure and keep the fumes in the tailcone.
The tube-within-a-tube was well seated. It appears that the fuel was flowing around the small plastic line (not pictured)
Instead of using plastic lines, I went to a 3/8 aluminum line. Bent it into a useful shape.
The upper end was easily installed. Make sure that the tubes are aligned with the tee.
I pulled the rivet for the plastic line and installed a snap bushing.
Works good now. You may want to check for shrinkage of the small line, and make sure that the tee is still pointed up so that the standpipe function is still effective. I have about 400 hours on the airplane and it has been in service about 2 1/2 years. Add a year or two for the initial installation of the vent lines.