Fuel system leaks in paramotors

by Had Robinson
updated December 4, 2021

Pilots will not see fuel leaking out of the lines going from the tank to the carburetor because there is always a negative pressure in the fuel system.  If there is a leak, air will enter the system instead of fuel leaking out.

Do you see bubbles moving through the fuel system with the fuel?  They are most likely bubbles of vaporized fuel due to negative pressure on the gasoline.  Aviation fuel has the lowest vapor pressure and motor gasoline, the highest.  The higher the vapor pressure, the more likely fuel will vaporize when subject to negative pressures.  The lower the fuel tank and fuel level, the greater the negative pressure on the fuel in the lines.  These bubbles may form in the fuel lines, particularly at the higher parts of the system where negative pressures are greater.

Gasoline formulated for winter use has a higher vapor pressure (and costs less to produce).  If this grade of fuel is used in summertime, it can turn into bubbles inside your fuel lines and carburetor quickly.

Whatever the cause of fuel vaporization, the bubbles can cause the engine to stall, overheat, or slow down due to "vapor lock" where the bubbles prevent the fuel pump from working.  A solution for this include flying in cooler weather or using gasoline formulated for the time of year you are flying.  Installation of the FSM also will permanently take care of this problem but it is not currently available (but very soon).

Finding the source of the leaks can be difficult.

Air leaks are just as bad as fuel vaporization as they both can cause a lean condition and your engine may fade at higher throttle settings and/or overheat.

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 (if the paramotor has one).

Fuel vaporization or an air leak?

If you think you have a leak, how can you tell whether it is a leak or fuel vaporization?  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, especially near the carburetor inlet, smear some Vaseline over the general area where the bubbles begin to appear.  If you still see bubbles, it is 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, it may be caused by fuel vaporization a.k.a. vapor lock.  MOGAS winter mixes in most states have a high vapor pressure which almost guarantees bubbles of vaporized fuel in the lines if you fly (as I do) in very cold weather.  When the bubbles get severe, vapor lock occurs and the fuel pump will quit working (it only pumps a liquid cannot pump a gas).  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 fuel vaporization is not causing a problem.  In this video, the vaporization got so bad that the engine quit.

Vapor lock is often incorrectly diagnosed as a carburetor problem.

The inline fuel filter may have what looks like air in it but it is almost always fuel vapor.

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.  Vaporized fuel (bubbles) in the carburetor *will* stop the liquid fuel from going into the engine.

Defective inlet fuel filter

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 carburetor.  Some filters will leak around the internal cartridge where it abuts the top of the filter's plastic housing.  The nipples at both ends of the inline filter must be undamaged and the fuel lines clamped to ensure nothing leaks.

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.

Common areas of fuel system leaks

Pilots must be sure that the fuel line from the tank clunk to the fitting on the fuel tank does not leak at the top fitting inside the tank.  A give-away that this is the problem is the presence of bubbles in the fuel line exiting the top of the tank.  The closer (vertically) the fuel in the lines is to the level of the fuel in the tank, the less negative pressure there is.  This means that any bubbles seen in the lines just coming out of the fuel tank are air bubbles, not vapor bubbles.

Normally, pilots may see fuel vapor bubbles occurring at/near the fuel filter while the engine is running.  If the bubbles are not excessive, the impact on the engine may be minimal.  The only fix for this is the FSM.

A leak at the fuel line where it enters the carburetor inlet will *not* be visible.  It is very important that pilots make sure that this connection is 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.

Quick fix for vapor lock

Take a liter/quart of cold water and douse the carburetor and fuel tank.  This will cool things down and help control fuel vaporization.  For the long term, our FSM eliminates vapor lock and fuel vaporization.  AVGAS has a vapor pressure much lower than MOGAS but it will still vaporize if the vacuum on it is high enough on the fuel.

Troubleshooting the fuel system

Please this section of our Troubleshooting page for detailed information on paramotor fuel systems.

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