Occasionally a plugged fuel valve on a vintage Vespa scooter will need replacing. The repair is not hard to perform, but it does require one specialised tool and a little mechanical know-how. Once you have swapped the plugged valve with a new one and reinstalled the tank, the fuel should flow strongly and without the debris resistance that is common with old units that malfunction.
Loosen and remove the scooter seat from the scooter, using a socket wrench and sockets. Release the side panel hook on the engine-side panel and pull the panel up and away from the scooter. Set it aside for the moment. Follow the fuel line to the carburettor. Use a flathead screwdriver to loosen the fuel hose clamp on the carburettor.
Pull the fuel line off the carburettor. Redirect it to an empty, portable gasoline container. Open the fuel valve to full flow and drain the gas tank. Wait for it to drain completely. Close the container. Use the socket wrench and sockets to remove the fuel tank retaining bolts. Pull the tank up and out of the scooter frame and fish the fuel line into the scooter frame as the tank moves upward. Leave the frame gasket on the tank.
Open the top tank lid. Insert the fuel tap tool into the tank through the opening and line up the wrench end with the nut inside the tank. Twist it loose -- it may take a good hard twist. Loosen the nut until it comes off and pull the tap out of the bottom of the tank. Clean up any gasoline residue with a shop rag.
Insert a new fuel tank from the tank bottom after temporarily removing the retaining nut. Insert the retaining nut into the top of the tank and hook it onto the copper pipe sticking out of the top of the fuel tap inside the tank. Wiggle the tap so the nut falls to the bottom where it screws on to the tap body. Use the fuel tap tool to tighten the nut onto the tap after positioning it in the right direction. (The actuator pin faces the front of the fuel tank.)
Attach a new 1/4-inch rubber fuel line to the bottom of the fuel tap at its spigot. Use a flathead screwdriver to tighten a new clamp onto the line after inserting it onto the spigot. Carry the fuel tank over to the scooter and fish the fuel line through the scooter frame cavity. Pull the hose through to the engine side as you lower the tank into the frame and seat it properly on its frame gasket.
Pull the fuel line to the carburettor and cut off the excess hose with a pair of snippers. Pour some gas into the tank and hold the remaining fuel end with a rag. Let the gas flow for a second to confirm it will come out of the hose. Turn the valve off. Place a new clamp on the hose and attach it to the carburettor spigot. Tighten the carburettor-side clamp with the screwdriver. Reattach the seat to the frame after tightening down the tank to the frame with the socket wrench. Reinstall the engine side panel and attach the retaining hook. Take the scooter for a test drive after filling up the tank.
You can obtain 1/4-inch fuel line, as well as hose clamps, at any automotive parts store for a very low cost. Vendors sell the hose line by the foot and a full line replacement requires no more than three feet.
Never repair a fuel valve or work with gasoline near a flame or heat source. The gasoline fumes can ignite quickly, causing an explosion.