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Leaking fuel tank caps and problems priming the fuel system

by Had Robinson

New, leaking tanks are a result of poor quality control and inadequate inspection by the factory.  The edges of tanks where they contact the caps and the openings for the various tubes may not be properly finished.  The result is that the tanks leak not only fuel but are also difficult to pressurize with one's breath.  The net result is that pilots have trouble with priming the fuel system.

There is nothing wrong with the concept of a priming system using a tube to pressurize the fuel tank.  A primer bulb, on the other hand, adds weight and is another unnecessary point of failure in your fuel system.  In addition, most priming bulbs are not resistant to the effects of ethanol in fuel and will quickly harden, crack, and leak – just like many other fuel system components.

Testing your fuel tank for leaks

The best way is to immerse it in water (just the top of the tank) and blow in the primer tube to see where the leaks are. If there are leaks do the following, as appropriate:

  1. Cap Openings:  With a wide file, finish the tank openings until they are perfectly flat and smooth.  A block of wood (so it's flat) with sandpaper may also be used.
  2. Caps:  Be sure to check inside the caps carefully.  I have discovered defects UNDER the cap gasket that caused leaks (see the photo below).
  3. Tube Openings:  With brake cleaner or mineral spirits, thoroughly clean the tube inserts and their openings.  Using RTV ("blue") from the auto parts store, LIGHTLY coat the inside of openings and the outside of the tubes and insert them into their respective openings. If you use too much RTV it might plug the openings/tubes that need to be clear, like the primer tube.  Make sure the primer tube is lined up properly.  Let things sit for a day in order to thoroughly cure.
  4. Retest your tank for leaks. Make sure the primer tube is not restricted or blocked with RTV or bits of tank material.  If it is, you can use a length of wire to clear it.
  5. Another way to discover leaks in the fuel system is to just fly the engine a dozen hours.  Dust in the air will stick to the oil mixed in the fuel where the leaks are.  There are often multiple leaks in the same tank.

The photo below is of a defective Miniplane fuel tank cap.  Pilots would never suspect that the reason their fuel tank is leaking and unable to easily hold priming pressure is because of a molding defect that is under the cap gasket.  There is a sealing ridge in the base of the cap that goes completely around it.  The red arrow points to (1) of (3) gaps I found.  The other (2) gaps are not as big as this one but will case leaks, just the same.  I looked at other Miniplane caps in our shop and they also had this defect, some more, some less in depth.

The only fix for this is to smear RTV all over the UNDERSIDE of the cap gasket.  You will have to have a very fine pick to lift out the gasket.  Do not get RTV on the TOP side of the gasket.  If it smears around the cap where it is visible, a shop towel must be quickly used to wipe off the excess.

defective Miniplane fuel tank cap 

If all is OK, reassemble everything. Without leaks, the tank will easily pressurize and the carburetor will prime correctly.  If you want your engine to start the first time, every time, check out this tip on properly priming a paramotor.

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