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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 large bubbles in the fuel line going into your carburetor?  They are most likely vaporized fuel.  Gasoline, especially when it is formulated for winter use, has a low boiling point (around 110+ degrees F) and if this grade of fuel is used in summertime, it can turn into fizz 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).  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 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?  One easy way is to pressurize your fuel tank (as when priming the system) while your engine is idling.  Connect a flexible tube to the priming pipe on the gas tank.  This way, you will be able to observe the fuel system much better.  If the bubbles do not disappear after a while, you know that you are looking at vaporized fuel (small bubbles or fizz).  If the bubbles disappear, you have an air leak because with positive pressure in the fuel system instead of the normal vacuum, air is not going to be sucked into the system.  Do NOT use an air hose to pressurize the fuel tank, it could blow up and you could make the news....

Another way to determine whether it is an air leak or vaporized fuel is to do the following.  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 coming in to the system at some point, you can do the simple test below to determine whether it is a leak or fuel turning into vapor.

At the point where the bubbles start, smear a wad of Vaseline over the general area.  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 air and/or 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 "boil" 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.  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.