04/24/13

by Had Robinson

Miniplane and other manufacturers have trouble with properly finishing their fuel tanks and connections after they are molded. Not sure why this is.  It would take them a minute to fix it and then test the tank for leaks.  Oh well....

In particular, they fail to smooth out the edges of the tanks where they contact the caps and the openings for the various tubes. The result is that the tanks leak not only fuel but they are difficult to pressurize (with one's breath) = trouble with priming. At least half of the Miniplanes, for example, I have ordered have fuel tank leaks.

There is nothing wrong with the concept of a priming system using a tube to pressurize the fuel tank  A primer bulb, however, adds weight and is another unnecessary point of failure in your fuel system.

First of all, make SURE your fuel tank does not leak. 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 (and there will be), do the
following:

1. With a wide file, finish the cap openings until they are perfectly flat and smooth.
2. With brake cleaner or mineral spirits, thoroughly clean the tube inserts and their openings.
3. 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 you will plug the openings. Make sure the primer tube is lined up properly.
4. Let things sit for a day in order to thoroughly cure.
5. Retest your tank for leaks. Make sure the primer tube is not restricted or blocked with RTV or bits of tank material.

If all is OK, reassemble everything. Without leaks, the tank will easily pressurize and the carburetor  will prime. When priming, it helps to rotate the engine so that excess fuel will run out of the carburetor and not into the engine and flood it.