The Mikuni fuel pump is the primary
pump used in the ultralight industry, When mounting the pump make
sure that it is no more than 11 inches away from the impulse outlet on the
engine. Make sure that it is mounted away from any heat source, and that
it is positioned so that the vent on the pump face is facing down.
line used to connect the pump to the engine impulse outlet is of a much
thicker wall thickness than regular gas line, and is not effected by heat.
The gas line and vacuum lines are generally exposed to sunlight in most
older configuration of ultralights and it is suggested that they be
checked on a regular basis for cracks, discolouring, or UV deterioration.
All lines should be carefully clamped, with clamps that do not pinch the
The fuel system should have some
kind of a water trap, this can be as simple as a loop in the line used to
join the two gas tank halves together on a root tube mounted tank, a
proper inline filter with a drain cock, or a pickup line located an inch
from the bottom of the tank. tank.
During initial installation of the
tank it should be inspected to make sure that no plastic is present,
inside, from the drilling of holes, or from the manufacturing process.
More than one pilot has had to land with an engine out only to discover
plastic lodged in his fuel pump.
A fuel filter should be installed
in the fuel system prior to the pump to prevent any foreign material from
entering it. If for any reason the filter is removed and is going to be
reused make sure that the direction of flow is the same as when it was
removed. Several pilot has reported reinstalling their fuel filter
backwards, depositing all the foreign material collected directly into
their pump and carburetor. Most quality filters have arrows showing the
direction of fuel flow.
When considering fuel tank
location, try to make it as close to the engine as possible with the least
amount of vertical draw. Although the Mikuni pump has been known to draw
fuel vertically over 4 feet the recommended distance is only 18 to 24".
Make sure all tanks are properly
vented. Many ultralight manufacturers use a standard marine fuel tank,
with a vent on the handle that can be screwed in to seal for
The problem here is that the vent
can vibrate closed, or can be inadvertently left closed. This allows the
engine to run for a short period of time before it runs out of gas, just
about the time your 50 feet in the air.
If you have one of these tanks a
solution is to drill a small hole in the handle, at its highest point.
Another problem with some of these tanks is that they have a steel check
valve located in the tank fuel outlet, in some cases this ball is to heavy
for the Mikuni fuel pump to lift completely, causing a fuel starvation
Make as few restriction in your
fuel system as possible.
Some pilots have installed
electric fuel pumps into their systems, in addition to the Mikuni pump.
There are several potential problems with this.
First the electric pump is capable
of producing considerably more pressure than what the fuel system is set
up for. Secondly when the pump is turned off it is an added restriction.