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Fuel system leaks

by Had Robinson

Air leaks in your fuel system can cause your engine to fade at the higher throttle settings and/or burn up so they must be fixed.  These leaks are not to be confused with leaks in the fuel tank.  The former are leaks in the fuel system piping/filters that allow air to enter the system.  Fuel is stored in a tank below the engine.  A pump on the carburetor sucks the fuel up and then pumps it into the metering system of the carburetor.  If there are any leaks in the piping or filters of the system, the pump will suck air instead of fuel.

On the other hand, external leaks in the fuel tank are more an annoyance (the oil in the leaked fuel attracts dust/dirt).  These leaks make it more difficult to prime the fuel system because it is harder to keep positive pressure in the fuel tank when the pilot blows on the priming tube.

Do you see bubbles in the fuel line going into your carburetor?  They are most likely vaporized fuel.  Gasoline formulated for winter use has a lower vapor point (around 110+ degrees F) and if this grade of fuel is used in summertime, it can turn into bubbles inside your fuel lines and carburetor.  This condition can cause your engine to stall (a.k.a. "vapor lock").  The only solution for this is to fly in cooler weather, use gasoline formulated for the time of year you are flying, or use aviation gasoline (AVGAS).  You can also take a liter/quart of cold water and douse the carburetor and fuel tank.  This will cool things down and help control fuel vaporization.  See our fuel/oil specifications page for a detailed discussion.

Fuel vaporization or a leak?

If you think you have a leak in your system, be sure the weather is cool so bubbles of fuel vapor are not as likely to form.  Rarely, the bubbles may be caused by a leak somewhere in your fuel supply system.  How can you tell if it is fuel vaporization or a leak?

With the engine at idle, look carefully at all the parts of your fuel system from the carburetor to the fuel tank.   If you see bubbles appearing in your fuel system at some point, smear a wad of Vaseline over the general area where the bubbles are.  If you still see bubbles, it is just fuel vapor.  If the bubbles suddenly stop, you know you have a leak.  Vaseline that has been stored in your freezer works the best.

IMPORTANT NOTE: The fuel filter always has what looks like air in it but it is just fuel vapor (OK).  This will not affect your engine's performance in any way.

The Top 80 fuel pump has to suck fuel from the tank below.  The lower the fuel level in the tank, the greater the vacuum needed to get the fuel to the engine and the greater the tendency is for the gasoline to vaporize and form bubbles in the lines.  Vaporized fuel can vapor lock the fuel system and cause fuel starvation, a potentially serious condition that can ruin an engine.  Pilots who use aviation gasoline (AVGAS) will not experience vapor lock as easily because AVGAS is of superior quality and consistency (and is why it is nearly double the price).  For the details on how to eliminate lead fouling from the use of AVGAS (and the increased maintenance this requires), see our fuel specifications page.

Defective fuel filters

It is important to use either the OEM fuel filter or the WIX #33001 in your system.  Cheap fuel filters do not remove the very fine particles that will clog the pump inlet fuel filter screen inside the carburetor.  Some filters will leak around the cartridge where it abuts the top of the plastic housing.  The nipples on the ends of the filter must be perfect or they, too, can leak air.

TIP: To easily remove the fuel filter from the tubing, heat the tubing with an air gun or a hair dryer until it is almost too hot to touch.  The tubing will easily slip right off of the fittings on the fuel filter.  Use a slight amount of pure silicone grease on the fittings and tubing to ease installation and prevent the tubing from splitting.

vulture