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How they work…by
TLRMan Nov2004
Granted, they are slowly becoming a thing of the past, but it’s a
good read…
This entry is to cover carburetors. There are several types, Smooth
Bores, "CV", and Flat Slide, to name a few.
The terms "Down Draft", and "Side Draft" are relating to the
position of the carburetor. The Down draft is angled so the air
flows Down into the engine. As for the side draft….Well, it’s
mounted on the side.
What the function is, of any type, is to supply the engine with the
proper Air/fuel ratio, regardless of engine speed.
Most carbs, can have their internal parts changed to suit almost any
type of engine performance, such as different camshafts, higher
compression ratios, etc.
The most commonly used today, is the "CV" carburetor. "CV" means
Constant Velocity. It’s job is to keep air velocity entering into
the engine, the same at any give rpms. It also gives manufacturers
an easy time of setting the engine up to meet emission standards.
So, how does it do that?
Let’s briefly cover Air/Fuel ratios….
If you are to run gasoline in an engine, the proper Air/Fuel ratio
is 14.7 to 1. Meaning 14.7 parts of air, to one part fuel. This is
the "Stochiometric air fuel ratio". What the heck is that you ask?
It is the exact air fuel ratio to completely combust a fuel to water
and carbon dioxide. It doesn’t really get there, but It’s close! If
you want to run Methanol, the ratio is 6 to 1, We will cover the
differences of fuels in another post.
With this in our heads, the manufacturer of the engine has to figure
out, how much air, the engine is going to pump through it at max
rpms, and all those in between.
Let’s Do It!
We must take the capacity of one cylinder, times the total
revolutions per minute, divided by 2. This will give us the volume
of air used per minute, in a four cycle engine. Don’t divide by 2
for a 2 cycle engine. This is considering each cylinder has it’s own
carburetor.
Now back to the Air/Fuel ratios…
Once we know how much air an engine flows, (pumps) per minute in CFM,
one cubic foot of air weighs .081 pounds, or one pound of air equals
12.4 cubic feet, we can now determine how much weight of air, in
lbs, that we have per minute.
Note: if any of you at home wish to figure this stuff out.. To
convert Cubic Centimeters to Cubic Inches, Divide CC’s by 16.4 to
get Cu.in.
Then divide by 1728 to get cubic feet.
Does the term "CFM" (Cubic Feet Per Minute) sound familiar to any of
you? Well, if you play with cars, a Holley 600 CFM 4 barrel
carburetor flows exactly that!
Next, is to establish the weight of gasoline, which is almost 8
pounds per gallon. So with this in mind, using the 14.7 to 1 ratio
of air to fuel, we can actually determine how many pounds of fuel
per minute we will need to make the engine run. This will be applied
as 14.7 pounds of air to 1 pound of fuel.
Fuel jetting selection can now be determined and be applied to the
application.
Now, don’t you wish you were paying attention in Math Class???
So now we know what we need for fuel requirements, how do we control
it at different rpms?
We build a carburetor!
What an engine does, is lower the air pressure inside, which allows
atmospheric pressure to "push" air into the engine.
15 pounds per square inch of pressure.
I bet all of you though it Sucked???
It has been found, well, actually proven, if you pass air through a
tube, and take another smaller tube "Spray Bar" and position it
perpendicular to the airflow, it will produce a pressure drop across
the small tube. If you put this small tube into a container that
holds fuel, it will draw the fuel up the tube, and spray it into the
airflow. So we put a large fuel jet, let’s call it the "Main Jet"
(piece of brass with a hole in it that feeds a certain amount of
fuel per minute), and the engine will run! AT FULL SPEED !!!
Ok, so now we have to figure out a way to throttle the engine down,
so it won’t blow up. Let’s put a slide in front of the airflow, this
way we can limit how much air goes in…less air, less engine speed.
But now, the fuel jet is blocked off, and the engine won’t run!
Hmmmm…. Lets drill another hole, and put another tube in, just ahead
of the slide on the engine side, but put a smaller fuel jet in,
"Pilot Jet" (we don’t need as much fuel at idle), and then either
add another hole to feed air, "Air Jet" or configure the slide to
let a little air through. Ok that works, so the engine idles now,
but poorly. Lets put a special screw, "Low Speed" that controls how
much fuel that goes through the smaller jet, that can get into the
engine at idle, a couple of turns, either in or out, and our engine
idles nicely.
Now, BISH grabs a handful of Throttle, The slide moves up, to let
more air into the engine, and the engine makes a gurgling noise,
blows black smoke out the pipe, and all of a sudden, it revs out of
sight! We are missing something. We can control the idle, and we can
control top speed, but we can’t control the middle of the rpm range…
I got it! Let’s put something into the slide, that fits into the
tube that the bigger fuel jet is screwed into, "Needle and Needle
Jet". Now, in order to vary how much fuel can come out, as the slide
moves up, we have to put a taper on it. So, the slide goes down, the
taper is bigger, thus blocking off the fuel from the big jet. As the
slide moves up from BISH cranking on the throttle, the taper is
smaller, allowing more fuel to enter into the engine! The engine
now, transitions from idle to midrange, to full throttle, with ease!
If we raise or lower the needle in respect to the slide, we can
control how well the engine responds in the midrange of it’s
operation.
We have ourselves a Carburetor!
Now, we want to control how much fuel and air is going into the
engine just a little better, for performance, and emission
considerations…..
Lets put a Throttle plate in front of the slide, and attach a rubber
diaphragm to the slide. The Throttle plate opens, the engine speed
increases, vacuum pulls on the diaphragm, and moves the slide up to
allow more fuel and air through.
When the engine idles, it requires less volume of air, but there is
a certain velocity of air moving through the carb.
As the engine speed increases, by opening the throttle plate, the
diaphragm pulls on the slide, and keeps the air velocity the same.
There is more volume of air, but the velocity will stay constant.
By doing this, we can keep the airflow and fuel delivery, into the
engine, constant, no matter what speed the engine is turning.
So, if you crank the throttle wide open, the slide doesn’t move
until the velocity is there to support it.
This is how a Constant Velocity Carburetor works.
I hope you find this as entertaining as it was for me to write it.
Mark "TLRMan"
What part of the carburetor controls what engine speed.
Closed to 1/8 throttle: Air screw (if carb has one) and Pilot Jet
1/8 to 1/4 throttle: Air screw, Pilot Jet, and Throttle slide..
(some have cut-outs in them)
1/4 to 1/2 throttle: Throttle slide and Needle Jet
1/2 to full open: Needle jet , Main Jet
 (AndyW - I purloined images from elsewhere on the Net and
forgot to make a note of the original locations. Apologies to the
originators!)
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