Fuel system leaks

by Had Robinson
updated December 27, 2020

Pilots will rarely, if ever, see fuel leaking OUT of the lines going from the tank to the carburetor.  This is because there is always a vacuum in these lines so air leaks IN to the system instead of fuel leaking OUT, even when the engine is off.  However, air leaking IN will cause a lean condition and may cause your engine to fade at the higher throttle settings and/or burn up.  These leaks must be fixed!

Fuel supply system leaks are not to be confused with leaks in the fuel tank.  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 may be 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 will quickly 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 best of all, 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?

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 any point, smear some 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.

If you experience engine stall at or near full throttle, be certain that it is not caused by vapor-lock!  MOGAS winter mixes in most states have a HIGH vapor pressure which almost guarantees vapor lock if you fly (as I do) in very cold weather.  Attaching a Go-Pro or other camcorder that can view the fuel lines while the engine is running is a good way to be certain that vapor-lock is or is not the problem.  Vapor lock is often incorrectly diagnosed as a carburetor problem.

Note: A leak at fuel line where it enters the carburetor inlet will NOT be visible!  It is very important that pilots make sure that this connection is air tight.  Some paramotor manufacturers put a clamp at this junction.  If there is no clamp and you want to be sure there is no leak use a 2.5mm (3/32") zip tie as a clamp.  The tie MUST be double wrapped around the fuel line in order to work properly.

The inline fuel filter may have what looks like air in it but it is just fuel vapor.  If does not have a continuous stream of vapor bubbles, it is not a problem. 

The fuel pump has to suck fuel from the tank below in nearly all paramotors.  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 of the gasoline to vaporize and form bubbles in the lines.  Vaporized fuel can vapor-lock the fuel system and cause fuel starvation and, often enough, engine stall.  Pilots who use aviation gasoline (AVGAS) will not experience vapor-lock as easily because AVGAS is of superior quality and has a vapor pressure that is a fraction of MOGAS vapor pressure (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 can clog the inlet valve filter screen inside the pump chamber of 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.

Turkey Vulture