Hi Bill,
Here's how we did it on our Gulf 1075 recreation. Note: the car now has a single filler cap on the right cowl, as per late Mk1b (Mirage) chassis. So it had to be single fill, yet vent and fill both tanks. The CAVs don't like taking the fuel very fast as the necks aren't vented during fueling. So a few things were done at once:
1) RH filler neck fitted with a vent tube inside the main filler pipe so air can escape during fill. Otherwise as the fuel spills past the elbow in the filler neck air cannot escape. It`s the neck not the tank that needs venting during fill. Venting the tank will not prevent fuel sloshing in the neck from backing up because of trapped air in the neck below the elbow. This vent tube is closed to the atmosphere when the locking filler cap is put on. The Monza style cap is retained for looks but serves no purpose in controling fuel fill or tank venting. These are not air tight (or fuel tight) caps.
2) A crossover vent line runs over to the stub filler neck on the left had tank to let air escape from that tank as the fuel level rises during fill. This is also capped by the locking cap.
3) Both tanks had vent lines fitted to the top service plates with integral rollover valves in case things go real bad. These lines were fed to the Vapour Separator.
4) A crossover feed line - balance pipe was run between the tanks. Either in tank fuel pump could run the whole system.
CAV tanks are vulcanised ballistic nylon bladders with foam to prevent collapsing the tank and reduce sloshing. It doesn`t completely stop it. The locking filler cap we use is one way vented to prevent cavitation of the tank, however venting to prevent pressurising is done by the vent valves and lines to the Vapour Separator.
5) A Vapour Separator was made from the cut off left hand filler neck. This sits on top of the left fuel tank. The separtor has a closed off bottom and top through which only the large filler neck vent tube passes. While driving any fuel sloshing up that tube can only travel to the right filler neck, not to the atmosphere. No fuel goes up this vent tube during fueling. There is a chamber between the top and bottom plates that is only open to three smaller tubes. One from each bladder tanks rollover vent and one vent to atmosphere (more on this later). The vent to atmosphere tube exits the top of the part, internally this tube is coiled in a verticle spiral so any fuel that managed to slosh up the vent tubes does not flow up the vent to atmosphere tube.
The vent to atmosphere tube will not have any fuel in it, this is important as you don`t want to be redirecting fuel with the vents. Also a charcoal cannister won`t last long if exposed to liquid fuel.
There are solenoid valves that manage the purging of charcoal cannister to the engine on production cars. This requires a computer, intank pressure sensors, wiring and a whole lot of co-operation among parts that probably never worked in a GT40 application before. Until I get my head around that one, we`ll be venting to atmosphere through a charcoal filter to keep the smells down. Running the vent line to the PCV or intake manifold might work but it`s an unknown. Would be nice to know if anyone out there has a donor car purge system we can use.
Hopefully you`ll glean some benifit from the example.
Cheers
I have some Mocal rollover units seen here: Breather valves
