Must Read Carburetor Info
Posting as requested by Author (named at end of article):
So, You want to buy a $1000.00 Carb?
Carburetors are possibly one of the greatest enigmas on an internal
combustion engine. All too often one of two things happens when racers
address the carb they are using; they either pay too much or too little
attention to it. There were many responses to a reply I authored on
“Race-Ready Carbs” a few months back, and thought I would expand more upon
One of the most common mistakes made in racing is over carburetion. Bigger
is not necessarily better. Engines are evaluated on volumetric efficiency
(VE). Most street engines are about 50% efficient, meaning a 302 cubic inch
engine is only flowing about 150 cfm at the median operating RPM, say 2500.
A full tilt, 358-inch NASCAR engine can generate amazing numbers in the
range of 125% volumetric efficiency, or about 450 cfm at 4500 RPM. Based on
everything the GCR allows, I believe a good estimate for a well-built,
“legal” A-Sedan engine would be 80% volumetrically efficiency. Holley
Carburetors offers a formula for determining which carburetor size to use
for naturally aspirated engines. For conversation purposes, I will offer it
Engine CID X max RPM
The “.3457” represents 80% volumetric efficiency. For a .040 over 302
operating at 7000 rpm, this would equate to 623.66 cfm. Now, Holley further
states that drag racers should add 50 cfm to this formula, and road
racers/circle trackers/street machines should subtract 50 cfm. The GCR
limits us to 4776 Holley’s and according to Holley; we should be using
573.66 cfm based on the parameters. So? We are basically slightly over
carbureted already. Believe it or not. Those of you bouncing your engines
to 8000+ RPM (as I have been told by some CFR guys) will argue this point
and with good reason. That RPM should require about a 700 cfm carburetor,
due in part to the mid point VE being nearer to 4000 RPM. I do question
these claims though, because based on the limitations of the GCR with regard
to head porting, I do not see how the engines can breathe at that RPM given
the port and valve size and camshaft lift…legally.
Now, what does all this mean? It means you really do not have to pay as
much attention to your carburetor as you might think. It comes out of the
box pretty much preset; all you have to do is tune it. For you die
hards...YES you can squeak out more horsepower using a “race-ready”
carburetor from one of many different carb shops, but I firmly believe that
you can find more horsepower in other areas of the engine for far less
money. Example- How many of you took your distributor out of the box, stuck
it into the engine, set the timing, and nothing else. What if I told you
there is as much as 15 hp under the cap that is unused due to improper
tuning? More on that in another article. All this first ‘chapter’ was
meant to explain is that a 4776, 600 CFM carburetor is actually a real good
choice for A-Sedan.
First, you have to determine at what RPM your engine will idle. With that
done, the age-old formula works for the idle air screws. Screw them in until
you hear or feel the engine stumble, then back off a quarter turn. If you
have four corner idle, keep going around the carb until you know you have it
right. With the divider in the intake plenum, this should be fairly easy to
do. It is far more difficult with an open plenum intake, but A-Sedans do
not have that concern. Now, if your adjustment is more than three full
turns away from bottoming out, you may want to consider modifying the carb
by drilling 3/32” holes (one each) in the trailing edge of the front
butterflies. This will allow more air into the engine at idle without
opening the throttle plates too far with the idle speed screw on the base
plate. If you have the idle adjusted too high without the modification, you
are actually past the idle circuit and will notice little to no effect when
adjusting the idle air-bleed screws. Too much idle speed adjustment (without
the holes) can also cause the motor to run lean at idle, because you have a
controlled vacuum leak and not enough fuel. This can cause engine to
overheat by merely idling, and many racers think it is because of lack flow
of air through the radiator. Additionally, you are about 10% or more into
the accelerator pump “cycle”. (More on that later), reducing the accelerator
pumps effectiveness. Remember, anytime you change the idle speed, you should
also adjust the idle mixture and accelerator pump preload.
In the thread a few months ago, I expanded my (and several carb shops)
opinion on power valves. I remove mine and plug the resulting hole. Here
is why- A power valve actually has little to do with power. It may be
better referred to as a “gas saving valve” The principle of it is to keep
the air fuel ratio leaner during moderate engine demands, thereby saving
fuel. When the vacuum signature falls below a certain predetermined point,
a spring within the valves overcomes the vacuum keeping it closed and opens
it, allowing more fuel to flow into the venturies. What determines this
point is the tension of the spring. Holley marks them numerically. “6.5”
is an example. When the vacuum falls below 6.5 inches of vacuum, the valve
opens. You can tailor the valve by purchasing valves with different set
points and in certain racing applications it may actually benefit you. I
personally believe you are better off removing it and here is why; Because
of the inherently poor vacuum signature associated with radical camshafts,
the signature WILL vary, causing the valve to open and close erratically.
Additionally, since there is a port that leads directly into the venturies,
a backfire of the engine can rupture the diaphragm of the valve. The
easiest way to determine on your car if this has occurred is to place a
vacuum gauge to the vacuum port located high on the passenger side of
metering block. If there is any vacuum at that port at idle, either the
valve is blown, or your engine is already below the set point of the valve
and it is open. If you do opt to remove the valve and plug it, you must
also increase your jet size 6 to 8%.
There are two main ways to ensure that your carb is jetted properly at the
track, and you can actually (and I recommend) using both. First is a
broadband O2 sensor, which should typically be between 11:1 and 13:1 on a
racing application. If it is higher, you need to enlarge your jets by
replacing them with a higher numerical (bigger) jet, richening the air/fuel
mixture, and vice-versa. The second is via a plug check. The only real
accurate method to do this is by running the car for several laps and then
shutting the engine down at the end of the longest straight after being at
full throttle in high gear. Coast or get towed back to your pits and pull
the plugs as soon as possible and see what they look like. What you are
looking for is a uniform, tan-grey color. Black (or darker) means you are
too rich, while white (or lighter) means you are too lean. Re-jet
accordingly. Plugs can tell you a great many other things and I would
recommend searching the web for pictures that will illustrate for you what
is going on inside your engine. I personally think too many racers spend
too much time re-jetting carbs. MAJOR changes in altitude, air
temperature, or barometric pressure should be the only reasons you re-jet
once you have determined your optimum jets sizes. There should not be more
than a four number difference between your front and rear jets with your
front numbers being smaller than the rear. If you have had the fortune to
be on a engine dyno with an EGT sensor on each header tube, you may have
determined that you need four different jets sizes, My experience is that
the Edlebrock Performer RPM series of intake manifolds flow well enough to
make it unnecessary, since the variance is most often associated with
differences in airflow between individual intake runners. It’s more common
on tunnel ram or open plenum intakes.
Fuel Pressure -
Another common “myth” is to set the fuel pressure as high as the needle and
seat will withstand. Wrong!!! I read a great tech article that explained
it like this. Have you ever placed you thumb over a garden hose and
released it? The result should have been an initial, aerated “pulse” of
water. The same principle applies in fuel pressure. The initial fuel pulse
causes aeration (much like a fuel injector) in the bowls, momentarily
reducing the actual amount of fuel passing through the jets. To solve this,
replace the stock needle and seat with a larger one, mainly one’s used for
alcohol carburetors, and turn the pressure down to 4½ to 5 lbs. or lower.
Higher pressures only seem to work in applications that have sustained,
wide-open throttle (WOT) like NASCAR super speedways and NHRA Classes. Do
not confuse “pressure” with “volume”. The GCR does not limit us on the size
of the fuel lines or fittings. Personally, I use ½” line through out, along
with #8 fittings and hoses. If the GCR would allow it, I would use a
return style system as well. Your fuel pump should flow at least 140 gph
and if the GCR would allow, I would run a voltage limiting device that
monitors engine RPM and adjusts the voltage to the fuel pump accordingly
(example; 3-4 volts at idle, and full voltage at WOT)
And finally, the mystifying accelerator pump-
Now to reiterate my earlier post response to accelerator pumps. First, you
must understand what it does. The accelerator circuit compensates for a
sudden loss of vacuum signature by introducing more fuel to the venturies.
When the throttle is closed, the engine develops base-plate vacuum. This
vacuum draws fuel through the main jets. Once the throttle plates are
moved, a vacuum loss occurs, and the accelerator circuit pumps fuel through
the center squirters.
In ANY engine, it is preferable to maintain a constant air/fuel mixture
regardless of volume. Modern fuel injector systems are best suited for
this, with the many monitoring devices such as the MAP Sensor (Manifold
Absolute Pressure) which sense vacuum loss, Throttle Position Sensor (self
explanatory) MAF, (Mass Air Sensor) etc. that makes constant, real-time
changes to the air/fuel ratio via a central computer. If A-Sedan used them,
I would be encouraging you to get a lap top with a flash program. Instead, I
will you tell you that a carburetor will NEVER be able to duplicate this.
So that leaves us with a tuning our accelerator circuits in an attempt to
maintain that air/fuel ratio.
The two adjustments you have are how much fuel is introduced, and when it is
introduced. The two tunable components are the discharge nozzles and the
cams. Of those two, the cams should be focused upon more than the nozzles.
Based on my experience, the nozzles should be either #28’s, or #31’s. Some
are long tube nozzles; some appear to have no “tube” at all. I have noticed
little difference between the two. All 4776 have come with 35cc pump houses,
vs. the 50cc pumps that are available on larger 4150 series and 4500
Dominators. 35’s are more than adequate as they are capable of delivering 5
to 6 full discharges without replenishment. As stated in the previous post,
too many racers make the mistake of tuning the accelerator circuit by
revving the engine from idle in the pits or their shop/garage to diagnose
it. The problem with that is there is no load against the engine, so it is
not an accurate representation of the dynamics the car will be under on the
track. The best way to tune is adjusting the cam in response to what the
car is doing for a certain point on the track. An Example: You are
negotiating a second gear turn and the car stumbles and sputters momentarily
when you roll back into the throttle. Again, if you were confident this is
not being caused by a lack of ignition timing, the problem would indicate
that you have too much fuel for the load vs. RPM. To resolve it, reposition
the accelerator pump cam to point where the cam is thinner in relation to
the given throttle position. Remember this rule; If you want more fuel at a
given point of throttle, you need a thinner portion of the cam ramp and vice
versa. A thinner position means that the pump is less compressed and has a
greater distance to travel and therefore delivers more fuel. Naturally, the
opposite applies as well. Holley Trick kits come with numerous cams that
are identified by color, and each cam has two positions it can be mounted
in. Therefore, the variance in accelerator cam pump “timing” has multiple
I wish I could give you magic formula for determining the best
profile for your engine. To my knowledge, there is not one. I will tell
you that peeking over your competitor’s fender will do you no good either
because you may not drive like them, weigh the same, have the same ignition
timing, gearing, etc. Trial and error, along with anything this tech
article may have helped you understand better, will be your only remedy.
Putting the air cleaner back on….
As the saying goes, If you cannot dazzle ‘em with brilliance, baffle ‘em
While I promise you there is no BS in this article, (although there may be
philosophical argument) clearly there are numerous other functions the carb
performs. If you want to learn more, there are a number of really good
books on the subject. Metering plates, emulsion tubes, air bleeds, annular
discharge boosters etc all play a vital roll in carburetor performance.
However, while they can be replaced, they cannot be tuned. IF anyone
reading this retorts that they bought a carb from Quik Fuel, Barry Grant,
The Carb Shoppe, etc and noticed a substantial difference, I would respond
by asking what was wrong with their original carb that made that big a
difference. You may have noticed a similar difference with a new
out-of-the-box 4776. I would also offer that unless one of those high
priced manufacturers is present for hands-on technical tuning, you still
bought an out of the box carb built for a wide envelope of performance
applications. Don’t get me wrong, When I eventually move up to SPO or GT1,
and have Yates heads and a 9000 redlines, there will be a killer,
aftermarket carb on top. I suppose I should actually get my AS on the track
for the first time. I have raced (other forms of motor sports) for almost
25 years and built them all myself. I do not know everything, although I
may sound that way the more beer I drink. ASedan.net has helped me get to
this point on my car, so I thought I would try to help others. If I can help
anyone individually, please feel free to email me.
Thanks for reading,
Does thinner = leaner or richer? It seems to say both.
Great info. Thanks
Thanks for taking the time to write to artical. I use a wide band 02 sensor and log the data so that I can look at it. I do not know how people live without these things.
I agree with most of what you said but I have found that on some days the car does need a 2 or so sized jet change. The air changed a huge amount on the Sunday practice between the morning and the afternoon before last years Runoffs and I had to add two jets to get the mixture close.
Just my experiance.
Sorry Andy I see what you mean. Hey I can fix your carb, but I cannot type worth a damn. What I meant was...a thinner position with a less agrressive ramp angle on the cam. So that when you first press the pedal, not as much fuel comes out initially. Depending on the ramp angle, you can tailor it anywhere you want.
So sorry... mY proof reading sucks too!
Last edited by Walther; 06-12-2007 at 10:20 PM.
Phillip- Whatever works. Personally, I play with timing curves and plug heat ranges/gaps before I re-jet.
I had a lot of personal inquiring e-mails after my original post-reply. I thought this article might help.
Here's a question I have been wondering about:
Do wide band O2 sensors really work effectively with leaded fuel and if so is the life span of the sensor degraded because of the leaded fuel? For some reason I was always under the assumption that O2 sensors of any type weren't meant for leaded fuel.
I love articles that are "Just the facts Ma'am!" Great job.
I am just curious to know if there is any benefit in playing with the float levels. My experience has always used the float level as the basic adjustment that all of the other settings are adjusted from. Is the factory recommended setting actually the optimum?
Thanks for the facts.
Kopp- My experience has been that leaded fuel tends to shorten the life of the probe, but believe it does not affect the accuracy. Normally a bad probe will show very rich. Another option is an EGT (Exhaust Gas Temperature) sensor. They do not reflect change as quickly as O2 sensors but last a lot longer.
Bob- Raising the float level too high cause the emulsion tubes in the meter plates to be submerged too far which can harm mid-range torque. If more bowl volume is your goal, try bowl extensions. They are about the same thickness as a metering plate, but go between the bowl and plate. Much more common on alcohol carbs. I do not think it necessary on an AS engine. I use sight plugs in the bowls and try to maintain the fuel level right in the middle, which is slightly higher that the factory setting.
Everyone else. I did not plagerize anything (except for the formulas) in the article I wrote. This is what worked for me in the past. I have run IMCA Modifieds, Limited Sprints, NMRA Mustangs, NHRA Competition Eliminator and Superstock Mustangs, and even tuned Dennis Langstons ARCA Thunderbird Engine at Daytona in 1996. (he barely missed getting qualified, mainly because the car was pushing terribly) My automotive "fifteen minutes". I DO NOT KNOW IT ALL. I will probably get my ass handed to me my first time out. I was bored and wrote the article in an attempt to help. Clearly I need a girlfriend or something!!!!