There's a couple of things that'll cause fuel venting on the 74 classic, one is on refuel, cold fuel into the tanks (especially the reserves that will be full) will expand in the heat and vent, another is someone pulling the vol fill fuse (and forgetting to refit it
) to get round a tank shutting off early, and the other is a broken vent float valve in one of the tanks allowing fuel into the vent system and from there into the surge tank and overboard.