INSIDE JOB
For the first two possible culprits listed previously, the tank must be lifted out from the chassis to check these points. It’s quite simple, just remove the seat – five M7 bolts in this case (the seat hinge flap stops the front of the tank from lifting and two bolts hold the back in place). Turn the petrol lever to the ‘main’ on position – with the tap upright in this case – push the rubber chassis grommet away from the tap lever control rod and the tank can be lifted out (picture 1), rear edge first from the plenum chamber where it sits. There is enough fuel tube (600mm) to allow the tank, on removal, to be balanced at 90 degrees from its in situ position on the top of the chassis tank opening (picture 2). The petrol tap should be turned to the off position at this point.
If the tube is hardened and has cracked, this can now be checked and replaced. This, as it happened, was not our problem.The next item on the list is the fuel tap that rarely gives problems, but is a possible candidate when there’s a petrol leak. This tap is an example of a labyrinth valve, one that allows a flow of a fluid at given levels only when it is suitably positioned internally (in this case the ‘off’, ‘main’ and ‘reserve’ settings). The only repair, apart from swapping parts and replacement of its internal rubberised seal, is total replacement (picture 3). To access the seal the tap may be taken apart by removing the two screws either side of the control rod interface to the tap body. The control rod is held in place by a sprung ‘R’ clip that is simply pulled away from the movable valve switch by forcing it sideways. Releasing the two slotted screws as mentioned allows the removal of the ‘top hat’ mobile valve section of the tap mechanism; behind this will be found the circular valve seal that features three holes to, firstly, allow the free flow of fluid when correctly aligned and, secondly, to prevent leakage when in the ‘off position’ at this interface (picture 4).
In the case of the GS in question, the top hat was found to be fractured across its mating surface with the valve seal (picture 5). This must have been the problem and the cause of a leakage of fuel. Famous last words…
THIS IS WHERE IT ALL GOES WRONG…
A replacement top hat was sourced and the tap reassembled with its associated control rod and the tank replaced in the opposite order to its removal.
With the tap in the ‘off position’ the scooter was now petrol tight once more (or was it?). After turning the petrol on once more, the GS started with no fuss. Once again, and to our dismay, a dark stain appeared on the ground underneath. The old girl was still losing fuel and alarmingly badly.
THE CARB
Back to the drawing board so to speak. We were left with the final item on the fuel circuit to be considered – the carb. Admittedly, it was visibly wet-looking. This annoying feature is often seen on Vespa carbs, even with late models. This one was, perhaps, a bit too wet.
To work on any Vespa carb, the air filter box assembly must be removed (pictures 6 and 7). With the bare carb on display and, as it’s of a down draught design, a small ball of cloth was inserted into the choke mouth to prevent stray objects disappearing into the engine before attempting further work. Although the two slotted bolts holding the carb top to the carb body can be removed with the carb in situ (picture 8), it’s sometimes easier to remove the carb from the engine to work on it.
On removal of the carb top, the float and needle valve can be detached by removing the float’s brass pivot and retaining pin, so as to lift out the float and valve from the carb (picture 9). The float should be free of liquid ingress to work correctly. In this case, that float was not leaking or compromised and, for good measure, a new needle valve was provided for the reassembly.
In a perfect world we had covered every base as far as fuel leakage was concerned. However, on once again turning on the fuel there was still a leak. Focusing on the carb, petrol was visibly running from the joint between the carb top and the carb body and, following our process of elimination of possibilities of candidates for fuel leakage, another carb was cannibalised to try a replacement carb top. The leak was finally cured. The problem, or part of the problem, had been a warped carb top in addition to the compromised fuel tap. Unfortunately the GS160/SS180 Dell’Orto carb is make-specific and thus quite rare, so a spare carb had to be cannibalised for a spare carb top that was happily a non-warped item when fitted.
Problem solved – the old GS was no longer Juicy Lucy; it was now fuel tight and ready to roll!