Fuel has weight, a little less than .5 pounds per foot of
distance. A high wing Cessna has about 3-4 feet between the
fuel tank and the carb float. The float in the bowl is
raised by the liquid fuel in the bowl and by a lever,
presses the needle valve closed, stopping the flow of fuel
by gravity.

If the needle valve is leaking, there are two problems,
during engine operation, the fuel level will be too high
making the mixture too rich, although this can be manually
leaned, but the bowl may still fill to over-flow and this
will cause a lose of fuel which can result in an fuel
starvation accident.

When parked, it can cause a fire hazard and certainly waste
expensive fuel.

For a science experiment, take a straw and place it in a
glass of water and place your finger over the end of the
straw. When you lift the straw, the water won't flow out of
the straw because it isn't vented, but the pressure of the
water is due to the weight of the water caused by gravity.

Low wing airplanes and some high wing airplanes with carbs
also have fuel pumps, all fuel injected engines have pumps
because gravity is not powerful enough to operate the
injection system.


--
James H. Macklin
ATP,CFI,A&P

Just to explain this, ATP is Airline Transport Pilot
CFI, I have Gold Seal Airplane SMEL and Instrument
A&P Airframe and Powerplant mechanic



--
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But government sees it as an obstacle to be overcome.
some support
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"Morgans" wrote in message
...
|
| "Jim Macklin" wrote
in message
| news:II7Tf.118045$QW2.26850@dukeread08...
| #2 Full tanks increase fuel pressure, a poor seal on
the
| carb float needle valve will leak and fill the bowl,
which
| then over-flows. The tanks have about 10-15% free space
| even when full to allow parking on non-level surfaces
before
| the tank vents begin to drain.
|
| Hold the phone, here.
|
| You mean fuel tanks can build up pressure with functioning
vent lines?
| Hummm.
|
| If it is a bad (plugged) vent, then I'm not going to fly
it until *that*
| detail is fixed.
| --
| Jim in NC
|