Basic Carburetor and Fuel System Maintenance and Tuning Tips
|There is no substitute for a
well-designed fuel and air delivery system.
By ignoring these two critical areas, all the work ofbuilding
a strong powerplant is wasted.
|For maximum horsepower, the coolest,
most dense air possible should
be available at the carburetor inlet. Keeping restriction in the
inlet path to a minimum - or better yet, pressurizing the air - is
|The denser the air, the more you can
get into the cylinders. This
allows the engine to burn more fuel and make more power.We
recommend that a hood scoop or outside air intake should be installed
wherever rules allow. Under hood air is heated by the engine
and headers and reduces the amount of power that can be produced.
A reduction in temperature of 10 degrees F. is approximately
equal to a one percent power gain.
|There should be a minimum of three
inches of clearance between
the top of the venturis and a hood scoop. If an air cleaner
is installed, the tallest possible element is preferred with four-inch
element preferred for racing engines over 500 HP.
|When a hood scoop or external air
intake is used, it is highly suggested
that the carburetor be sealed to it. Otherwise, air willflow
across the top of the carb and out of the inlet tract rather than
into the air horn. If air is forced past the carburetor it cansiphon
fuel, causing the engine to run lean. Windshield snorkels are
especially notorious for siphoning unless the rear is sealed. Air
pan kits for sealing the carburetor to the scoop are available or
they can be fabricated. An air bell or radiused intake should be used
whenever possible to increase air flow into the carburetor.
|It is not unusual for a drag race
car to improve ETs by 0.3 second
and increase top speed by as much as seven miles an houafter
installation of a sealed scoop. A car will not pick up ET after
the scoop is sealed off if the scoop is too short or the fueldelivery
system is inadequate.
|On oval track cars, the same is
true. Paying attention to the inle tract
design will pay off. Depending upon track length, ovaltrack
cars will typically improve lap times by 0.1 to 0.5 second once
an optimized air intake system is installed.
|Many racers experience fuel delivery
problems without ever being
aware that something is wrong in their race car’s fuelsystems.
Today’s state-of-the-art engines produce a lot more power
than a race engine of ten years ago. The process ofproducing
horsepower revolves around the conversion of fuel into
energy. The more pounds of fuel an engine can burnefficiently
per hour, the more horsepower it produces. Even though
your car may not miss, pop, bang, skip or do anythingelse
peculiar, it may not be getting all the fuel it needs to make maximum
|In oval track applications, a BG
Belt Drive or Hex Drive Fuel Pump
is preferred where use of a mechanical fuel pump isspecified.
These pumps offer the highest fuel delivery volume of any mechanical
pump yet maintains low fuel pressure at lowengine speeds.
This feature alleviates “loading up” of the spark plugs.
The BG Six-valve and Super Speedway mechanical fuelpumps
will also deliver ample fuel volume when used according to
race cars, a BG400-2 Electric Fuel Pump is the best way
to guard against fuel starvation. If a car is “lazy” or “laysdown”
at mid-track then pulls well in a higher gear, the engine may
be experiencing intermittent fuel starvation.
|Why? Typically, the carburetor bowls
are full at the starting line so
the car leaves hard but in the process, drains the bowls dry. In the
lower gears, the car accelerates rapidly with the engine picking up
rpm very quickly. This rapid acceleration increases the demand for
fuel. When the float bowl fuel level drops, the car “lays down” because
of fuel starvation. In high gear, engine speed increases more
slowly allowing the bowls to fill again.
The Fuel Can Test
|There's been no shortage of well-researched and well-written
articles and books explaining the workings of the fuel system.
However, many racers, both novice and experienced alike do not
fully understand the physics of fuel flow and horsepower. To
produce torque and horsepower requires a mixture of air and
fuel. To produce 1-horsepower for 1-hour requires approximately
.5-lbs of gasoline. If you ran a single-cylinder engine, like
the one in your lawnmower, under a load of 1-horsepower for
1-hour and weighed the fuel tank before and after, the tank
would weigh approximately .5-lb (five-tenths of a pound)
lighter. Therefore the equation for fuel flow is 1-H.P. = .5-lb
of fuel, per hour.
|This is expressed on a dyno sheet as B.S.F.C. (Brake
Specific Fuel Consumption). Highly-tuned racing engines can
sometimes by more efficient, yielding B.S.F.C. figures of around
#.40 which means 4-tenths of a lb of fuel, per h.p., per hour.
Incidentally, the formula for Alcohol is approximately 1-lb of
fuel, per h.p. per hour which, as a consequence, necessitates
the running of a belt-drive pump, but that's another story.
|Typically, a 600-HP engine will require 300-lbs of gasoline per hour
and, by the same formula, an 800-HP engine needs 400-lbs per hour.
Remember, these quantities of fuel have to be delivered past the needles
and seats and the fuel pressure regulator. Consider also, the fuel
delivery system has to combat 'G' Forces: loadings that are so
formidable they can threaten to stall the fuel in the line (this may
also give a clue as to why a fuel line that is too large in diameter can
be as harmful as one that is too small). This leads us to the area that
is least understood.
|When you have only one carburetor it should be easier to feed than
two, right? Wrong, in an engine with a tunnel-ram layout, both the
needle and seat area and the float bowl capacity have doubled! Whereas
the single four-barrel car that is most prevalent today, has a much
harder task in keeping the fuel bowls full! A 700-HP tunnel-ram engine
needs 350-lbs of fuel per hour which equates to a little over 85-lbs
per float bowl. A
700-HP engine running a single four-barrel (not so uncommon these days)
per float bowl, compared to a 1200-HP Pro Stock engine with
demands of 600-lbs max, 150-lbs per bowl.
|So what happens if fuel delivery is weak? Your engine may not miss
or "burn-up" parts. It may just not perform to expectations. The new
camshaft, racing-carburetor, or flowed-heads that didn't pick-you-up may
have overstressed an already taxed fuel delivery system. Carburetors
cannot disperse the optimum air/fuel mixture unless the fuel system has
the ability to maintain correct float bowl levels. Fuel levels that are
two low may not cause the motor to miss or "burn" a piston, but they
will reduce fuel flow and performance will suffer. It is not uncommon
after upgrading a fuel system with a single four-barrel carburetor to
pick up 1- to 4-tenths of a second. In extreme cases, E.T.s have been
known to decrease by as much as 1 second!
|Can a fuel system that is too large hurt performance? No, it assures
your combination will reach its full potential: the needles and seats
will shut when the float bowls are full. Conversely, if your fuel system
is marginal, fluctuations in battery voltage will cause fuel flow
changes from run-to-run which affects the float levels in the
carburetors and out-the-window goes your consistency! So how do you know
if your volume is adequate?
|Test your fuel system by obtaining a 1-gallon gas-can (do not use a
moulded-plactic gas container, or marked super-jug, or antifreeze-jug as
you will not get accurate readings). Open up the top of the tin-can and
insert the two or four carburetor fuel lines from your regulator, switch
on the system and carefully measure the time it takes to fill it. High
10-second cars will need to pump 1-gallon in 25-seconds or less. A
9-second car should fill the can in 20-seconds or less, 15-seconds is
all it takes for an 8-second car and under 12-seconds for 7-second
Note: It's essential to observe two strict rules during the test. One,
keep a fire extinguisher handy and two, do not carry-out the test by
|1/4 Mile E.T.
||Time to fill a
1-Gallon Gas Can
||under 12 seconds
|How do you know you're getting all of the performance from your car?
Perform the gas-can test even if your car is running well - you have
nothing to lose and everything to gain: including the prospects of
improving your E.T. and gaining increased consistency.
|When your car isn't performing, always carry out the gas-can test
first - it's one of the least-expensive diagnostic aids you'll
encounter. Keep in mind that valve springs, ignitions systems, torque
converters, even engines have been changed, when all the time the fuel
system was at fault.
|So, my car has failed the test - now what do I do? Call us at BG
Fuel Systems: we have the correct fuel pumps, regulators and plumbing
for your particular H.P. level. Rely on The BG Powerfuel Advantage to
help put you in the winners circle.
|For the same reason, only filters
specifically designed for racing,
such as the BG5000 or BG Inline filter, should be installed.
Use of a filter is strongly advised as long as it doesn’t restrict
fuel flow. The fuel filter should be installed in the line before
the fuel pump. This filters the fuel, preventing any damaging
material from entering the fuel pump or the rest of the system
Controlling Fuel Pressure Settings
|Fuel pressure should be set between
6 and 8 psi for a gasoline carburetor.
An alcohol carburetor is a different animal with very different
requirements. The Alky carburetor will require 4 to 5 psi at
idle and 9 to 12 psi at wide open throttle.
|Remember, fuel pressure is not a
substitute for volume! If the fuel
bowls are not full, the pressure is meaningless. In fact, fuelpressure
is simply an indication of the amount of restriction in the fuel
Regulators and Bypasses
|Most electrical fuel pump systems
require the use of a fuel pressure
regulator. One BG regulator is sufficient in manyapplications.
The use of two regulators is recommended when using
other types of regulators or in high horsepower engines to avoid
excessive fuel restriction and provide adequate volume.
|With mechanical fuel pumps, and some
electrical pumps, a bypass
is preferred rather than a regulator. A diaphragm bypasswithout
an idle bleed is recommended when constant fuel pressure
is needed from an electrical or mechanical pump. A beltdriven
fuel pump, using gasoline or alcohol, requires a diaphragm
bypass with an idle bleed. Higher pressure mechanical
fuel pumps delivering alcohol, such as the 15-psi BG Six-valve,
require a throttle bypass to supply the variable fuel pressure
required by the carburetor.