How to Repair a Gas Leak in a 50cc Scooterby Chris Gilliland
A fuel leak on a 50 cc scooter is usually caused by a problem between the fuel tank, fuel hose or carburetor, if not all three. In most cases, leaks are caused by a damaged fuel tank, a broken fuel valve seal or a damaged fuel hose. The carburetor can be affected by debris in the fuel supply, which prevents the carburetor's internal valve from closing and causes the excess fuel to overflow. Likewise, the float arm that controls the valve can be improperly adjusted and allow more fuel than needed to enter the carburetor. The exact cause of the problem, however, must be determined before attempting any repairs.
Locating the Leak
Park the scooter on a level work area, using the scooter's center stand. Unlock the seat and raise into a fully opened position. Remove the screws from the bottom of the under-seat storage compartment, using a screwdriver or a ratchet. Pull the seat and the attached compartment out of the scooter.
Loosen the clamp attaching the air filer housing to the carburetor, located directly above the engine, using a screwdriver. Unbolt the air filter housing from the left side of the engine, using a ratchet.
Look for signs of leaking from the fuel tank, typically located at the rear of the scooter above the rear wheel, using a flashlight. Leaks will appear as an obvious stream of fuel or stains along the top of the engine below the fuel tank and often occur around the tank's fuel valve, located at the bottom of the tank. Remove the fuel tank, it is leaking.
Follow the hose running from the fuel tank to the carburetor. Look for pinholes, tears or cracks along the fuel hose. Loosen the clamps attaching the fuel hose to the fuel tank and the carburetor, then replace the hose if the hose is damaged in any way.
Look at the top of the engine directly below the carburetor for fuel puddles or stains, as well as drips or leakage from the carburetor overflow hose, the single hose attached only to the carburetor. Remove the carburetor, if it is leaking in any way.
Push the air filter housing over the carburetor, once you have identified and repaired the fuel leak. Bolt the air filter housing onto the scooter's frame and tighten the clamp to secure the air filter housing to the carburetor.
Repairing Fuel Tank Leaks
Remove the side body panels from the scooter, using a screwdriver or a ratchet. Turn the fuel valve to the "off" position or pull the vacuum hose off of the fuel valve if the fuel tank uses an automatic fuel valve. Unbolt the fuel tank from the scooter's frame using a ratchet.
Loosen the clamp attaching the fuel hose to the fuel tank valve using a screwdriver. Pull the hose off of the valve outlet using pliers. Cover the fuel valve outlet with a shop towel, then lift the tank off of the scooter.
Unscrew the fuel tank cap and hold the tank over a gas can and a funnel. Turn the fuel valve to the "on" position to drain the fuel tank. Alternatively, connect the valve's vacuum outlet to a vacuum source if the tank is equipped with an automatic fuel valve.
Inspect the fuel tank for physical damage, such as corrosion, pinholes or cracks. Also check the fuel valve mating surface for damage that could allow fuel to leak out from between the valve and the tank.
Remove the fuel valve and gasket from the fuel tank, using a ratchet. Remove the outer face plate from the valve using a screwdriver. Pull the valve lever off of the valve if you are working on a manually operated valve. Remove the rubber sealing diaphragm from the valve and check for tears or holes. Replace the diaphragm, if needed, then re-install the lever and faceplate.
Seal the fuel tank, using an ethanol-safe phenol novolac epoxy fuel tank sealant kit, if the tank has a pinhole leak caused by impact damage or light corrosion. Apply the sealant according to the instructions provided in the kit and let it cure completely. However, the tank must be replaced if the damage or corrosion is substantial.
Re-install the fuel valve onto the fuel tank with a new gasket. Re-install the tank onto the scooter. Reconnect the fuel and vacuum hoses to the fuel valve.
Refill the fuel tank with gas, then turn the fuel valve to the "on" position and watch for leaks. Alternatively, start the scooter's engine if the fuel valve is operated by vacuum. Replace the fuel tank and valve if the leak continues.
Repairing a Leaking Carburetor
Loosen the clamp attaching the carburetor to the engine using a screwdriver. Unplug the automatic choke electrical connector from the scooter's wiring harness, if any.
Turn the fuel valve to the "off" position, in the tank is equipped with a manually operated fuel valve. Pull the fuel hose off of the carburetor using pliers. Remove the vacuum hose from the side of the carburetor, if any.
Loosen the throttle cable adjuster lock nut, located on the side of the carburetor, using a combination wrench. Turn the cable adjuster clockwise until it is seated against the cable holder, then pull the cable end out of the carburetor throttle pulley. Turn the cable adjuster counterclockwise until the entire cable can be removed from the carburetor.
Pull the carburetor out of the scooter and hold it over a gas can and a funnel. Loosen the float chamber drain screw on the bottom of the carburetor using a screwdriver. Drain the fuel remaining in the float chamber into the gas can until the flow of fuel reduces to a light drip. Tighten the drain screw.
Remove the float chamber from the carburetor using a screwdriver. Check for corrosion or debris buildup around the float posts on the inside of the float chamber if the float is attached to the float chamber instead of the carburetor body. Clean the posts as needed using a carburetor cleaning spray.
Remove the float arm hinge pin from the carburetor body using a metal punch. Pull the float arm and the fuel valve needle out of the valve seat on the bottom of the carburetor. Remove the valve seat from the carburetor, using a ratchet, then place the seat in a container filled with a carburetor cleaning solvent.
Inspect the tip of the valve needle for rounding or damage. Ideally, the tip of the valve needle should be uniformly pointed. Replace the valve needle if it is rounded, blunted or misshapen.
Rinse the valve seat in warm water, then blow compressed air through the seat to clear away any remaining debris. Screw the valve seat into the carburetor.
Re-install the valve needle and float arm hinge. Turn the carburetor over, facing the bottom up, and hold it a 45-degree angle until the float arm is resting lightly against the bottom of the valve needle without compressing it. Check the angle of the float arm against the bottom of the carburetor. Ideally, the float arm should run parallel or at a slight angle toward the carburetor. Remove the float arm and bend the tang at the center of the arm with pliers, if the float arm is angled steeply toward the carburetor.
Re-install the float chamber and a new gasket onto the carburetor. Screw the throttle cable into the cable holder until it is completely seated. Push the throttle cable end into the throttle pulley. Turn the throttle cable adjuster counterclockwise until the throttle cable is tight. Tighten the cable adjuster lock nut against the cable holder.
Push the carburetor onto the engine, then tighten the carburetor clamp. Plug the automatic choke connector into the scooter's wiring harness. Push the fuel and vacuum hoses into place.
Turn the fuel valve to the on position and watch the carburetor for leaks. Alternatively, start the engine, if the fuel tank is equipped with a vacuum-operated automatic fuel valve. Stop the engine or turn the fuel valve off immediately if the carburetor begins to leak. Remove and recheck the carburetor.
- The Professional Motorcycle Repair Program: Fuel Systems, Volume 9; Professional Career Development Institute
- Haynes Scooter Repair Manual: Automatic Transmission 50 cc to 250 cc Two-Wheel Carbureted Models; Haynes Publishing
Things You'll Need
- Screwdriver set
- Socket set
- Fuel hose
- Gas can
- Fuel valve sealing diaphragm
- Fuel valve gasket
- Phenol novolac epoxy fuel tank sealant kit
- Combination wrench set
- Carburetor cleaning spray
- Carburetor cleaning solvent
- Air compressor
- Fuel valve needle
- Fuel valve seat
- Float chamber gasket
- Do not use a tank sealant that does not contain a phenol novolac-based epoxy, as these sealants are not compatible with ethanol-modified gasoline and will create clogs within the fuel system. Additionally, the fuel tank will develop leaks as the sealant breaks down.
- Do not ride your scooter or operate it for longer than needed to diagnose the fuel leak. Leaking fuel can ignite and cause serious harm.
An avid motorcyclist, Chris Gilliland has immersed himself into the two-wheeled world while balancing work life and raising three daughters. When he is not managing the parts department of a local, multi-line motorcycle dealership, Gilliland can often be found riding, writing or working on his motorcycle blog, Wingman's Garage.