If fuel starvation is to be avoided on a performance carb
installation some basic points need to be addressed. The fuel pump must be of
adequate size and there must be no restrictions in the fuel system that will
limit high rpm power output. A well set up petrol engine should require no more
than about 0.6 lb weight of fuel per horsepower per hour at peak power output.
One UK gallon (4.546 litres) of pump fuel weighs about 7.5 lbs so we can use
these rules to check that the fuel pump output is adequate. 100 horsepower
would therefore require 60 lbs or 8 gallons of fuel per hour - that's just over
1 pint per minute. Adding a decent safety margin to allow for wear and tear on
the fuel pump or jetting that is a bit on the rich side it is preferable to aim
for about 1.25 pints (0.7 litres) per minute for every 100 horsepower. There's
no sense in saving a few pounds by buying the smallest fuel pump available and
then melting the pistons by running weak at high rpm.
To test an electric fuel pump just disconnect the supply hose from
the carburettor and time how long it takes to fill a graduated measuring jug.
Remember though that this is testing the pump under a "no load"
condition. There may be further restrictions in the system in the shape of the
pressure regulator or the fittings in the carburettor fuel inlet such as the
needle valve in the float chamber. A better test would be to test downstream of
the pressure regulator and also to block the end of the fuel hose with a
drilled bush of the same size as the needle valve jet (or the combined area of
the several needle valve jets in a multiple carb installation)
This brings us on to the needle valve itself. Different carbs can
have a wide range of needle valve sizes fitted and this item provides the final
restriction on how much fuel can get into the float chamber. The flow rate
through a given needle valve will depend on the pressure that the pump is set
to deliver but we can set rough guidelines which should ensure adequate fuel
flow. The chart below shows how many horsepower EACH needle valve of a given
size will supply enough fuel for at normal fuel supply pressures. If there are
multiple carbs then remember that each one will only have to supply enough fuel
for part of the engine's power output - single carbs therefore require bigger
needle valve jets for a given power output than multiple carb setups.
|
NEEDLE VALVE SIZE (MM) |
BHP LIMIT |
|
1.50 |
80 |
|
1.75 |
110 |
|
2.00 |
145 |
|
2.25 |
180 |
|
2.50 |
220 |
|
2.75 |
270 |
|
3.00 |
320 |
Very occasionally the standard fuel pipe fitted to the vehicle
will prove too small if a very much more powerful engine has been installed.
More common is the problem where the pipe has been crimped partly closed
somewhere under the car due to accident damage. If the pump is theoretically
big enough but still not supplying enough fuel then it pays to inspect the pipe
run to the tank carefully.
Methanol fueled cars run at about twice the fuel/air ratio of
petrol fueled cars so the above limits need to be halved to ensure adequate
fuel supply.