Could be your tank breather pipes are blocked/damaged/missing.
The one that usually goes that no-one notices is behind the spare wheel bowl (inside the engine bay), if this is damaged it can result in siphoning od fuel from the tank that fills the battery tray & then drips onto exhaust (ask me how I know!!). There are also several connections that perish near to the tank that require removal of the firewall to get at.
Not sure if all early bays have the same tank breathers though - mines a 72.
Probably best to find a book haynes/bentley/etc and follow all the pipes and ensure they are all ok/fitted, often they are removed esp. if you dont have the stock air filter fitted.