So you have a nifty Weber DCOE or S&K Racing sidedraft carb,
set up for your vehicle, you're ready to chew gum and kick butt, and
you've run out of gum. You bolted on the carb in place of your SUs or
Zenith-Stromberg and voila! You have worse gas mileage, and the power
gain is slight enough to be psychological. What to do?
First, you must decide your target behavior. Do you want POWER,
TORQUE, or MILEAGE? A properly set up system will improve one or two but
probably not 3 of those items (you might win the lottery if it is
particularly misconfigured now, though).
Next, set your timing! Make sure your ignition system is good
from one end to the other. Get rid of those old fouled plugs, check
dwell and advance, and make sure nothing is intermittant. Set everything
to book values, unless you have a firm grasp of what's better for your
car.
One issue with Weber conversions is vacuum advance signal. If
your distributor took vacuum from the manifold, you should do the same
with the new manifold. You may have to modify it, or use a port intended
for a less important function such as pollution control (pollution
control mods should be done only for off-road vehicles).
If your distributor took vacuum from the carburetor, you should
probably just ignore it. The S&K Racing carb does offer a throttle-plate
vacuum port which you can try. A system with no vacuum advance will
require more initial mechanical advance than book values for
part-throttle tractability. Start with 5 degrees more, you may end up
with 10 degrees more.
Finally, set your idle mixture. Maximize RPM with the idle
mixture screws, then reduce idle speed to 850-1000 (lowest smooth idle
speed) with the idle speed adjustment screw. Tweak the idle mixture
screws again at this speed. Repeat until no further idle speed increase
is observed.
Your idle adjustment screws should be 1-1/2 to 2-1/2 turns out from full
lean. If they are far outside this range, your idle fuel jet is the
wrong size. Increase jet size to reduce the number of turns, decrease
jet size to increase the number of turns out from full lean (all the way
clockwise).
Road tests:
Use timed road testing to set advance. Stop
when you are at best time for the runs which reflect your desired
driving pattern. To reduce variability, use Wide Open Throttle
wherever possible, make your runs on level ground, and do multiple
runs in both directions. Please do all this in an open area with
good visibility, and slight or no traffic. The road which runs along
the river or coastline is a good bet. Consider that you want your
average speed over the course to be similar to or slightly below the
speed limit. That will attract the least attention and tie up the
least traffic. Note: This consideration is why 0-60 testing on open
roads is NOT recommended. Pick a narrower speed range.
Use a STOPWATCH, not a regular watch. You'll
need to record times to about 0.1 seconds. Use a buddy to time if at
all possible while you concentrate on driving in a consistent manner
over the same ground, and looking for other traffic! Try to run all
your timed runs in the same basic temperature conditions (10F
temperature degree change is worth about 2% change in air density,
and so mixture). Also, try to maintain a similar weight in the car
(with or without a timing partner, full gas tank vs nearly empty,
bags of cement or cases of Old Clem's Joy Juice in the trunk, etc).
If you are shooting for best mileage, your
tests should consist of sustained runs at the speed and over the
terrain you plan to use. Testing with a normal mix of highway and
around town can be misleading since your mind-set affects your gas
pedal usage. Changes of less than 2 or 3 mpg in a normal mix can be
ascribed to psychology as much as physics. So, you should fill the
tank, drive at least 1/4 of a tank at sustained speeds, and then
check the mileage.
Now you should have a set of benchmark
results for further use. Open up the access port in the top of your
carb, and remove and inspect the main jet assembly. The part/size
numbers are stamped on the side of each component. Record all
numbers.
DCOE Thumb rules
(use as a starting point):
Venturi Size: If your carb was indeed set up for a similar
sized engine, you should be OK on this. Increasing venturi size will
result in slower but greater airflow. This will move the power band
UP in RPM and reduce torque down low (for a given jetting). Let's
assume that your venturi size is appropriate for your engine. (Approprioate
venturi size is roughly proportional to the square root of
displacement x peak HP RPM. But it varies quite a bit with cam
characteristics and so is way beyond the scope of this article).
Main Jet: Starting point Venturi size number x 4. In the
case of the MGB, this should be 36x4 = 144 or a 145 main jet.
Air correction jet: Main Jet plus 60, or in the case of the
MGB, about 200 or 205.
Emulsion tube: We won't cover the emulsion tube itself,
as effects are too subtle for road testing (emissions control test
equipment would be more appropriate).
Idle jet: 50 for MGB (Mike, I need a thumb rule for this?
Altitude compensation: Reduce main jet size 0.05 for
every 5000 feet of DENSITY altitude. Density altitude is dependent
both on temperature and air pressure. Although calculating actual
density altitude is a fairly complex operation, you can approximate
it with actual altitude with this forumla:
Density Altitude = 75 feet * (Outside Air
Temperature F - (59F-Altitude/300)) + Actual Altitude
This is a crude measure and you may wish to
stick with Actual Altitude to avoid overleaning in warm weather.
To change behavior over the mid to
high range of RPMs (more than 1/6 throttle), change main and air
correction as follows:
For a given air correction jet,
a larger main jet will flow more fuel across
the RPM range.
For a given main jet, a larger
air correction jet will reduce fuel flow in the
higher RPM range.
In general, for racing, the air
correction jet will be smaller than the thumb rule above, and
the main jet larger. This allows for more fuel flow
over the whole range, and further enrichment at high RPM.
For economy, the thumb rules are close but may require small
adjustment. The question is, why are you looking for economy
instead of power at Wide Open Throttle?
Optimizing for power:
So, out we go to our test road. Warm up the
engine completely (I drive for 6 miles to get to my test road) and make
sure the air pressure in the tires is right, and fill the tank.
First, run another benchmark series to get a
baseline for current weather conditions. You should have enough practice
by now to evaluate your runs for consistency.
The main jets are the easiest to
work with, so we'll try them first. They are in the tip of the jet
assembly, and have a pointed "nose". Increase the size one step at a
time, and run a benchmark series. If you were running too lean, you will
notice a decrease in time spent above 3000 RPM. If you go too rich, the
sound of the engine will change dramatically, and times increase. Go
back to the next leaner size, and do another run to confirm the problem
and clear the plugs.
The air correction jets are in
the TOP of the jet assembly, and require that you
separate the threaded portion from the emulsion tube. Be gentle... grasp
the components to separate in PADDED pliers and wiggle them apart.
Twisting doesn't help much as these are press-fit and not threaded
joins. You will REDUCE the air correction jet (it has a flat nose) one
step at a time. This makes the mixture richer in the upper (4500+) RPM
ranges. If you go too far it will increase the times, and make the
engine sound odd in the upper part of its range... I got terrible
gargling and hesitation reducing from 190 to 180. (I didn't have a 185).
Switch back to the previous size and re-test to confirm.
Now we get into aspects of carburetor performance
which are transient or harder to quantify. We move away from the known
setting of Wide Open Throttle to gas
mileage and throttle response.
In highway cruising at legal speeds in most of our
vehicles, the throttle is mostly closed. This is because the gear is
high, the RPMs are relatively low, and the horsepower requirement is 10%
or so of maximum. This causes the main jet to be
shrouded and so most of the gas being used is coming through the
idle jet and progression holes.
Overall flow to idle port and progression is
controlled by the idle jet/emulsion assembly. Flow out
the idle port itself is further restricted by the
idle adjustment screw. Fuel is pulled from this port by
airflow through the main throat. It will be a somewhat
trivial portion of the fuel flow above idle.
Fuel flow through the progression holes
is significant above closed throttle but becomes less so after the edge
of the throttle plate clears the progression
holes. This is because fuel is drawn from the
progression holes by high-speed air flow past throttle
plate. As the throttle plate moves away from
the progression holes, less fuel is drawn from them. At
this point the main venturi is becoming unshrouded and
more fuel is flowing through the main jet.
During highway cruise the throttle plate is near the
progression holes. Changing flow through the
idle fuel jet and idle air holes will thus
most significantly change cruise mileage.
Modification would
involve:
1) Increasing the size of the idle fuel jet
may allow more power at a given throttle opening, increasing the
load at which the transition to the main jet
occurs. If you have large main fuel jets for good
power at higher RPMs, you will find more fuel economy at lower RPMs
by staying off the mains, and on the progression holes.
2) If indeed you have enough fuel to stay off the
mains, but still have low economy at cruise, you have either too
large an idle fuel jet, or too small an
idle air jet. Increase the size of the idle speed
air jet hole.
How do you
determine all this rot? You need to see what your
mixture is at your normal cruise speeds. The only way outside a dyno is
to do plug cuts after sustained operations at those speeds. This means
you turn off the engine and take it out gear after a sustained test at
speed. Coast to the side of the road and pull the plugs... comparing
color. You are looking for a chocolate brown or dark grey for all
results. White is too lean, and black is too rich. Light brown/grey is
also somewhat lean.
A note on plug cuts.
High-performance spark plugs such as Champion Gold and other high
temperature, anti-fouling plugs, can make it hard to read plug color.
You want to have the standard temperature range plugs installed for
thest tests. In addition, oil fouling will mask true mixture color. If
you get oil fouling on your spark plugs, fix >that
The following test assumes the idle mixture adjustment screw has been
properly set. The speeds are representative, and should be altered for
your cruise gearing and desired RPM (on the MGB this corresponds to
about 2K, 2.5K, 3K, and 3.5K RPM).
One method would be:
5 minutes at 35, check plug color
5 minutes at 45, check plug color
5 minutes at 55, check plug color
5 minutes at 65, check plug color
If the plugs are pale at 35 and 45, and go progressively darker at 55
and 65, your idle jet is too small, or your
idle air jet is "too large", or you are transitioning to the
main jet too early.
If the plugs are black at 35 and 45, and go towards brown or stay black
at 55 and 65, your idle fuel jet is too large for your
idle air jet. Increase the size of the idle air jet
hole. In extreme cases you might have to reduce the size of the
idle fuel jet (check the recommended size for your engine).
If your idle fuel jet is extremely small, you may be
already on the mains at 35 mph. This is unlikely. You would have noticed
this in the benchmark step when your idle adjustment screw
was several turns out from full lean.
When you have a relatively even color across the
speed/RPM range, fill the tank with gas and drive normally. With luck
you will have increased your part-throttle cruise mileage noticably!
Your mixture control will be better across the operating range and your
engine will be happier because of it.
----------- end of economy
optimization -----------------
Now we get into an even more subjective
optimization... Throttle response.
One complaint folks have about the Weber carb versus twin SUs is a "flat
spot" or dead zone, or bog, on throttle. Some tuners advise against the
Weber for this reason.
Well, now that you've optimized the mixture curve
over normal driving / racing parameters, you have probably virtually The missing piece of the puzzle is
the accelerator pump. This device should actually give
significantly stronger initial throttle response than a
constant-depression carb without one.
When the throttle plate opens
suddenly, airflow slows down suddenly. Bernoulli tells us that suction
in the venturi is caused by the air in the
venturi travelling quickly. When the air slows down, the
suction decreases, and less fuel is pushed into the throat
of the carb by ambient pressure.
In order to cope with the lean condition caused by more air and less
fuel, the accelerator pump squirts additional fuel into
the throat of the carb.
When you move the throttle plate
open, a linkage pulls on a spring. This spring moves a piston which
forces fuel into the accelerator jet chamber, and through the
accelerator pump jet, through a metering hole, and into the
throat of a carb. Fuel not used drains back into the
float chamber through a small drainback hole.
There are several things to consider about this squirt...
Volume, Pressure, and Duration. They are all interrelated but here's
how you can think of it:
If you increase the size of the metering hole,
Pressure Drops, Volume / unit time increases, and duration
decreases.
If you strengthen the spring, Pressure rises, Volume
/ unit time rises, but duration decreases.
If you decrease the size of the drainback hole,
Pressure is slightly increased, volume is increased, and duration is
increased.
So what do we do
now?
The
typical symptom on a book-value Weber conversion is a bog unless you
baby the throttle. This is because the mixture leans out when you try to
suddenly involve the main jets with insufficient
airflow. When you bring airflow up more gently, the progression
circuit helps you out, and the air is moving faster when you
get to demanding fuel from the main jets.
This shows that fuel volume is insufficent for
fast throttle movement. We increase accelerator pump
volume / unit time most easily by enlarging the metering hole (this item
is as easily accessible as the main and idle jets). Use a pin vise and a
set of drills 61-80 to gradually enlarge the metering hole. On the MGB,
I started about 77 and ended up at 69. More may be required as I get
used to the difference.
Try to reproduce realistic transitions, from
realistic RPMs, in all the gears. You may notice as you enlarge at some
point the initial response is good, then a short bog before things kick
in. This means the duration is too short.
If you see this problem at extremely low RPMs,
you may want to re-evaluate your requirements. Do you need to SLAM the
throttle from 1400 RPM? In any case, stop enlarging the metering hole if
you run across this problem. Going any further will require modification
of the drainback hole to provide a bigger fuel supply.
Intial Setup and adjustment of a
Pair of DCOE 40's on a 1500
Make sure that the CHOKE
lever is pushed in fully.
Check that the choke cable is allowing the enrichment levers
on the back of the Webers return to their stops. Loosen the clamps with a
7mm wrench and push the levers to their stops. Tighten the clamp screws.
Check the FUEL HOSES for
cracks (especially near the ends) or leaks.
Empty the water separator and
inspect or replace the FILTER.
Check for FULL THROTTLE
OPENING.
Have a helper step on the accelerator pedal while you watch
the throttle shafts rotate to the fully open position. Adjust the linkage,
if necessary. Make sure that the bellcrank under the carbs is not sticking
on its shaft.
Synchronizing the throttles.
Make sure that the throttles are synchronized. Remove the
brass plug (above the off-idle holes) of each carburetor section. Shine a
flashlight into each of the holes to see that the edge of each throttle
plate is aligned with the same hole when the throttle is slightly open.
Adjust the spring-loaded screw in the linkage between the two Webers. Make
sure that the Idle Stop Screw is NOT against its stop when you are making
the adjustment or you will be even more confused.
Check the FLOAT LEVEL.
Remove some fuel from the carbs and let the engine
idle for two minutes. Remove the wingnut caps.
Use the depth gauge
end of vernier caliper to measure from the top of the lower shelf to the top
of the fuel surface.
DCOE spec = 1.14 inch (29mm+4.5mm).
If the depth is NOT correct, the
top plate of the carburetor must be removed. Carefully bend the tab on the
float assembly that touches the needle valve. Reassemble and test the float
level as before.
Check for Air leaks
Check for Intake System AIR LEAKS while engine is running
Wiggle the carbs and pour water on the donuts and listen for a change in the
engine speed.
Adjust the IDLE MIXTURE.
Raise the idle speed to 1000-1500 rpm by turning the
Idle Stop Screw. Stop the engine. Remove the spark plug wires from the
two rear spark plugs and ground the wires. Turn each of the front two
Idle Mixture Screws clockwise and count the number of turns until the
Idle Mixture Screw touches its seat. Normally, this screw is about 3/4
of a turn from its seat.
Restart the engine. Adjust the two front idle mixture
screws so that the engine rpm is the highest possible. Allow a few
seconds after each adjustment for the engine to respond to the change.
Stop the engine. Reconnect
the two rear plug wires, disconnect, and ground the two front plug
wires. Adjust the two rear idle mixture screws.
NOTE: The idle speed should be the same when the
engine is running on the front two or on the rear two barrels.
Stop the engine and reconnect the wires. Turn the
Idle Stop Screw clockwise to the original position. Start the
engine.
Adjust the IDLE SPEED.
Adjust the idle speed by turning the Idle Stop Screw. If the
idle is too high when the Idle Stop Screw is NOT touching its seat, the
throttles are NOT returning to the closed position. Make sure that the
linkage is not binding.
Supplied to the manafacturers to be used as OEM carburation for each of the
cars listed, this table shows the factory jetting info as configured as 'stock'.
From Weber's own guides, the following table is a list of the standard
jettings
as supplied in their conversion kits. The kit #'s are also listed in the
table.
.....a project in process. data is in complete
and constantly being updated.......
From the eyes, ears, minds, and typing fingers
of the members of the Triumph Spitfire e-mail List, The Editors of the Triumph
Tune Catalog, with a special thanks to those people submitting photos and
jetting info that makes these pages possible, I bring you a compilation of all
the info I've been able to find concerning the initial setup of Weber DCOE
carburetors for the 1500 Triumph Spitfire (and now others...post
11/99).
There are many factors that can effect the initial settings that work
best for you. They vary due to driving habits, various engine modifications,
state on wear, etc ,etc.
Get your hands on a copy of 'Weber Carburetors' by Pat Braden,
ISBN #0-89586-377-4. It covers dis-assembly and rebuild, tuning, and
complete descriptions of all the sub-sections of the inner workings of the DCOE,
among others. Also the Haynes Weber Manual (?? ISBN #???)
is a very good source of info. A third source is the Weber Factory
Overhaul Manual (Weber part number #95.0022.35). You can order this
book from The
Performance Parts Warehouse Another excellent sources is
Advanced Performance Products
they carry a complete line of ram pipes, jets, air filters ands ancillaries for
the DCOE.
The Flow Paths of the Fuel
If idle jet size is correct, the idle
mixture screw (19) will be 1.5 to 2.5 turns out from full closed.
Progression ports,
......
Pump Jet, idle, and Main
Jet
The idle stop screw should have the
throttle plate (17)
sitting just at the edge of the first 'progression hole.
Just off idle and full speed cruising
WEBER CARBURETORS
A Technical Primer
By:
"If you have always wanted to know what makes
these carburetors so special, here's a basic, easy-to-understand
introduction to the world of Weber exotica."
Weber carburetors have been seen as
standard equipment on the finest racing and street machinery to
come out of Europe for over three decades. Maybe you've been
lucky enough to get a ride in a Ferrari or a Weber carburetor
289 Cobra; if you have, chances are, it's a ride you've never
forgotten! Weber-carburetor engines all have one thing in
common: they assault the senses with a rush of torque and a
brutal sense of urgency that is generally unmatched among
carbureted engines (and they have a sound all their own...go to
a Shelby American convention on Open Track day and you can pick
out the Weber-carbureted Cobras just by their sound; there is no
mistaking it!). The world's most beautiful, exotic and most
powerful engines have traditionally been fed through Weber
carburetors. But why Weber?
For starters, it's a modular carburetor
design. It is produced in a wide variety of styles which
incorporate different features, enabling the user to select
exactly the right design and size for the intended use. You can
even change its CFM to suit your needs, which should begin to
explain the Weber's superior adaptability for all kinds of
applications.
Now, if you're one of those people who has
always had trouble accepting the idea that the Weber is a
terrific street carburetor, consider it this way: Weber
carburetion is like an expensive musical instrument. If it is
not tuned properly, that instrument will never make beautiful
music for you-no matter what! And therein, lies the secret of
making beautiful music with Weber carburetors-initial
preparation...... It's what "tuned induction" is all about!
THE CONCEPT
The Weber carburetor was
designed to be totally adaptable to any size engine, for any
purpose, at any altitude. There is no such thing as taking four
of these out of their boxes and bolting them on to an intake
manifold...it simply isn't done that way. This carburetor was
intended for serious tuners and performance enthusiasts who want
the most that their engine can give them. Welcome to the Big
Time!
TERMINOLOGY
All Weber carburetors carry a
basic model number which is stamped at the base of the
carburetor on its mounting flange. The most well-known is the
good-old "48 IDA", a masterpiece of design and a marvel of
precision machining that has been around since the early 60's
with only minor revisions. In this case, the number 48 indicates
the carburetor's size. It tells us the carburetor has a bore
diameter and throttle plate size of 48 millimeters (about 1
15/16"), while the IDA suffix tells us that it is a high
performance downdraft carburetor. There is also a 40 & 46
IDA/3C. Again, a high performance downdraft, available in 40 and
46 mm sizes. The 3C means this one's a "3-choke" (the in-line
three barrel). The 40, 42 and 44 DCNF's are compact twin-throats
which feature a cold-start. As the prefix numbers indicate, they
are available with bore diameters of 40, 42 and 44 mm. Then
there are the sidedrafts - all Weber sidedraft carburetors carry
the suffix DCOE, their prefix numbers (sizes) ranging from 38 mm
all the way to 55 mm (that's close to 2 1/4"). So you see, all
those numbers and letters really mean something. It's all pretty
simple.....So, the next time someone mentions he's running
Weber's, ask him whether he's running DCOE's or IDA's and pick
up a few bench-racing pointers.
THE VARIABLE CFM FEATURE
Some where along the line,
you can probably recall seeing four 48 IDA's on a big, nasty rat
motor. You've also probably noticed that the same four 48 IDA
setup is used on 289 Ford engines, as on the Cobras, for
instance, You may have wondered how the same carburetor setup
could work on two such vastly different engines. It seems that
one engine would have to be either over or under carbureted, if
we assume that the carburetion is "right" on one engine.
Actually, this isn't true at all, because either engine is
running the same set of carburetors as the other. Assuming the
Webers are set up properly, the only thing the two systems will
have in common is their outward appearance. The Weber's most
interesting design feature is it's removable "choke" or venturi,
allowing it to be instantly converted from a large-CFM
carburetor to one of small CFM, or vice-versa.
By installing a smaller
choke, the carburetor is constricted and it flows less CFM, to
make it perform in the midrange, or to make it suitable for use
on a low-compression small block engine. Pull out those small
chokes, drop in some large-diameter ones, which may be nothing
more than thin-wall "sleeves", and you've got a set of 48 IDA's
that will flow enough CFM to make a big block scream. But don't
try putting those "big" carburetors on the small block motor! It
will fall flat on it's face, lack throttle response and become a
complete nightmare in traffic("....My buddy had a set of those
Webers on his engine, and boy! did that car run badly!!!!!"). In
order to get drivability, throttle response and lots of torque
from the Weber-carbureted engine, the choke size, therefore, is
the first consideration. How big is the motor, what's the
compression ratio and what do you want to do with it, once the
correct size choke has been selected for your application, the
jetting for all the rest of the circuits can be established
around that choke size.
THREE CIRCUITS
For the sake of simplicity,
let's look at the Weber carburetor as having three basic
circuits- the idle circuit, the accelerator pump circuit and the
main circuit.
The idle circuit is comprised
of two components, the idle jet and the idle jet carrier. With
these two pieces, the tuner can select exactly how much fuel and
how much air he wants to provide the engine at idle and during
the low rpm operation, while making very fine adjustments to
either, if necessary. The idle mixture is delivered as a
proportioned mixture whose total volume can be further regulated
with the idle mixture screw, which is located on the lower part
of each carburetor barrel. On a correctly-jetted idle circuit,
the mixture screw on a 48 IDA is never more than 3/4 of a turn
out. This will hold true 100% of the time, no matter what anyone
else tells you. If you have to go more than that, you'd better
heavy-up the idle jet. Even if you get it to idle, going more
than 3/4 turn tells you the jet is lean and you're going to have
other drivability problems, which brings us to the next part of
the idle jet's function.
The idle circuit in the Weber
isn't just an idle circuit - it does more than that. It
is actually the circuit which must carry the engine all the way
up to about 2,800-3,000 rpm, where the transition to the main
circuit take place. That means if you don't drive over 3,000
rpm, you're only running on the idle jets. After 3,000 rpm or
so, the idle circuit is entirely bypassed and no longer has
anything to announce. So, if you have a tuning problem that
"goes away" after about 3,000 rpm, that tells you to play with
the idle circuit. Or maybe the opposite is true. Either way,
it's very cut and dried as far as the two circuits are concerned
- so isolating the problem is a breeze.
One the most frequently
experienced "gremlins" with Weber carburetors is a seemingly
incurable and very annoying flat spot which rears its ugly head
at about 2,200-2,800 rpm. This condition is generally caused by
one of two things - you either have the wrong emulsion tube in
the carburetor, which is causing a rich stumble due to an
under-emulsified mixture at that particular rpm range or the
idle circuit is falling off too early to carry the engine up to
the point where the main circuit can take over, leaving a "lean
hole". In simple terms, the idle circuit is going lean too
early. Either condition is easily rectified. In the case of the
emulsion tube, there are really only a few which work really
well for V8 applications; and if you aren't using one of them it
is certainly a big part of the problem. If the flat spot is
still there even with the correct emulsion tube, then you'll
need to richen up the idle circuit. This is sometimes a tricky
area, because the first thing you want to do is throw in a
bigger idle jet, but sometimes playing with air bleeds, mixture
screws, or choke sizes can accomplish the same thing while
sticking with the original jet size. Seeking a little bit of
sound advice here can save a lot of time and hassle. The point
here is that these carburetors are designed to come off idle and
run smoothly all the way up. Your problems can be solved with a
little tuning on your own or by relating the symptoms to someone
who is knowledgeable enough to help you. Remember, these
carburetors will do just about anything you want them to, except
maybe wash your socks.
The accelerator pump circuit,
just like on any carburetor, is responsible for eliminating
"bog" and making a passing maneuver without a hesitation or
stumble. The circuit also has two basic elements. These are the
pump exhaust valve and the pump jet. The pump exhaust is nothing
more than a bypass valve and this is located in the bottom of
the float bowl. This is the piece that regulates how much fuel
you want to make available when you need that pump shot. Putting
a bigger bypass hole in the valve allows more fuel to bleed back
into the float bowl instead of out of the shooters. The smaller
the hole, the more fuel you're making available. You can even
put in a "closed" bypass for drag racing, when you need all the
juice you can get in order to get those slicks turning.
Obviously, there is nothing complicated about a simple bypass
system. The duration of the pump shot is varied by installing a
larger or smaller pump jet (shooter). Larger pump jets give a
heavy blast over a short period, while the smaller ones will
give a finer, longer-duration shot. As long as you leave the
bypass valve alone, you're still getting the same overall
volume. In most cases, the stock pump jets can be left alone.
The main circuit is the easy
one. This is where you make your power. This circuit has three
primary elements you should concern yourself with - the main jet
itself, the emulsion tube and the air corrector. You're thinking
that's a lot of pairs - usually, it's just a main jet. You know
how to "read" what your Webers can tell you on a road test, you
wouldn't have it any other way. The capability for fine
adjustment is what you pay for. Let's take a look at this main
circuit......
The main jet is stuck into
the bottom of the emulsion tube and sits in fuel. As the
carburetor begins to work, the main jet meters the amount of
fuel allowed to pass through it and up into the "main well"
around the emulsion tube. Air enters the top of the emulsion
tube through the air corrector which meters the amount of air to
be mixed with the fuel. The air blows out of the emulsion tube
through a series of holes along its length and aerates the fuel
that is rising up the well around the tube. This emulsified
mixture is then sucked out of the main delivery nozzle as the
"depression" in the carburetor increases to the point where it's
strong enough to pull it out. This occurs by 3,000 rpm or so,
and you're down the road like a shot.
Tuning the main circuit for
maximum power is something that can be done by a series of road
tests and a handful of jets. The simple rule of thumb for
jetting Weber carburetors is, if you want to implement a change
over the entire rpm range, you play with the main jet. If you
want to change the way the car feels at the high end, that's
where the air corrector comes in. Also, you should keep in mind
that the air corrector is a finer adjustment that the main jet.
Example: One step upward in the main jet (richer) equals about
the same as three steps down on the air (less air: richer). A
change of air corrector would be appropriate; for instance, if
the engine pulls strong to 5,000 rpm and then goes flat. This
would mean she's going lean on you up top; drop the air
corrector three sizes or so, and you'll probably be able to buzz
that engine right up to 7,000 rpm. If the motor feels sour all
the way up, go one or two sizes heavier on the mains only. No
magic! So, tell me, what's so hard about jetting these Webers?
STREETABILITY
Most people don't realize
that this carburetor, like all highly efficient items, is an
extremely simple design with very few moving parts. There are no
metering rods, power valves, rubber seals or plastic parts. The
accelerator pump on the 48 IDA is a brass piston. The throttle
shaft rides in a set of precision roller bearings. Webers use
brass floats, which cannot become fuel-logged, and gradually
sink with age. It is a superior example of precision machining
and "beautifully-fitting" components...it's really very unlikely
that one of these carburetors is going to "fail" and cause you
to be stranded somewhere. That's another reason why they're well
suited to street use and long-distance cruising - they are
extremely reliable.
With the infinite tune
ability of Weber carburetors, there is no need to compromise the
drivability or road manners of your car. If you know someone who
suffers from drivability problems with such a nice carburetion
system, he is doing so unnecessarily. A Weber unit should be
crisp, responsive and smooth. If it is not, something is wrong -
let's just say he's not through tuning it yet, that's all!
The first thing most people
notice when they go to Webers is an increased flexibility from
the motor. There is a natural tendency for a Weber-carbureted
engine to idle smoother, have a slicker "feel" to it a low
speeds (particularly if a hot camshaft prevented that feeling
before), and generally feel much more powerful throughout the
entire rpm range. This is largely because they use an
independent-runner manifold, which does not incorporate a
plenum. In a typical four two-barrel Weber layout, there is one
barrel directly feeding each cylinder without any
intercommunication between barrels or cylinders. This totally
"isolated runner" design ensures that each cylinder is fed
exactly the same as the next, without any chance of
charge-robbing or over-feeding. What you are doing, in effect,
is separately tuning each cylinder. This results in a dramatic
increase in horsepower output and torque in midrange, right
where street engines spend 90% of their time, making this an
ideal carburetion system for street use, where maximum
flexibility creates greater driving enjoyment. The throttle
response with an independent runner induction system is also a
new experience, it's second to none. A Weber carburetion system
will respond like a fuel injection unit, with which it shares
some similarities: short runner length, isolated design and
essentially a low fuel mass to move when you hit the throttle.
(Remember, you're not asking that cylinder to gulp all the
mixture from that big plenum area - that's a lot of mass, by
comparison. The only mass to move is what's in that one short
runner). The main difference between fuel injection and Weber
carburetion is that one relies on fuel being injected under very
high pressure, while the other responds to the needs of the
engine via the depression principle. For street use, the Webers
have the edge - it's what they were made for.
In the mileage department, it
really depends on the rest of the engine and your driving
habits, but 16 to 18 mpg is not unusual on the highway. This is
pretty respectable, when you stop to consider that the engine is
fed by all eight barrels constantly. There is no such thing as a
progressive system here. Another thing: Webers will run happily
on regular gas. If you can run regular now, you can continue
doing so after installing the Webers. This is purely a function
of compression ratio and ignition timing, not induction. In
fact, if you're running a 10.5:1 engine, you may find it's a
little bit fussy about which brand of fuel it wants. giving you
detonation at times. Generally speaking, the Weber carburetion
will likely change this for the better, suppressing the tendency
to "ping". One reason for this phenomenon is that the fuel
distribution is now fully controlled, eliminating the "lean
spots" which sometimes are present in conventional manifolds
which distribute fuel from a central plenum. Lean cylinders run
hot - excessive cylinder heat means detonation.
TUNING AND MAINTENANCE
A Weber carburetion system
will not be right, unless it's synchronized to ensure that each
carburetor is doing exactly the same as the next - the name of
the game is perfect cylinder tuning. The synchronization
procedure can either be a breeze or a nightmare, depending on
whether you have a well-designed linkage system or not. The
secret to a good linkage setup is that it must allow independent
adjustment of each carburetor without affecting all the rest as
you go through the procedure. Here again, if someone tells you
they're absolutely impossible to synchronize, you might study
his linkage. Chances are, it's incorrect and he's fighting
himself. The right components are now available to take this out
of the dark ages.
The final idle mixture
adjustment on each barrel is a simple adjustment which is
performed by ear, but because there are four carburetors,
a lot of guys feel intimidated. It's done the same way you do a
single four barrel, except in this case, you can listen to each
cylinder separately. It may take you four times longer, but it's
no more difficult at all. Each mixture screw, as it is turned,
will have a noticeable effect on engine rpm, as the wrong
setting will cause the cylinder to "go away" - it's just like
pulling a plug wire. No matter how hard you try, you can't mess
this up if you remember one thing: always start from scratch at
3/4 turn out. From there, you go 1/8 of a turn either way and
it's usually in, not out. This will get you out of the woods if
you ever get lost.
Once the unit is synchronized
and the idle mixtures are dialed in to give you the smoothest
possible idle, you can hang up your Unisyn and screwdriver til'
next spring, because now it's set! And when it's set,
it's set!! They will not suddenly "go out" on you and ruin
your day at the picnic.
EXPENSE
When it comes to the Price of
Admission to "Weberdom", what can one say...... Webers are not
for everyone. This type of induction unit represents a sizeable
investment. It's still possible to put a unit together on your
own with bits and pieces, and if you're a fast-lane spender, you
can opt for a ready-to-run unit created especially for your
engine. Dollarwise, Webers usually fall into the category of a
supercharger with carburetors. The price of opening up a box and
pulling out a science-out Weber unit with all the right pieces
and associated hardware will run you from $3,000.00-$4,000.00,
depending on how much flash you have to have. Sound expensive?
Maybe not, if you consider that all things are relative. When
you figure the price of a top quality paint job at $5,000.00 to
$10,000.00, a completely redone interior at $3,000.00 to
$5,000.00, a set of trick wheels and tires at maybe $3,500.00,
and your basic "nice street engine" at $3,000.00 to $5,000.00,
another two grand for an item that changes the car's whole
personality falls right in line.
And as they say, "the fun's
in the driving". Weber carburetion is a lot more than something
that's exciting to look at. Every time you take that machine of
yours down the road, you become more aware of your engine's
ability to do everything it should do with a minimum of fuss.
Throttle response, quick acceleration and overall flexibility
are the constant reminders of what you've spent your money on -
the ultimate carburetion system!
40DCOE2
(from Weber General
Catalog 1/5/1961)
NORMAL
OPERATION:
The fuel
arrives through the needle valve (1) to the bowl (4) where the
float (3) controls the opening of the needle (2) in order to
maintain a constant fuel level. Through the ducts (6) and the
main jets (5), it reaches the emulsioning tubes (12) from which
after having been mixed with the air coming from the air
corrector jets (11), through the pipes (10) and the nozzles (7)
it reaches the carburation area consisting of the auxiliary
Venturi's (8) and chokes (9).
STARTING DEVICE
The fuel
flowing from the bowl (4) arrives to the starting device through
the ducts (32) and the starting jets (30). Emulsified with the
air coming from the hole (29) it reaches the valves opening (35)
through the ducts (31) and definitely emulsified by the air
entering from orifices (34) is then carried by means of the
ducts (33) to the carburetor throats below the throttles.
Engine cold
starts: starting device inserted (position A)
Engine starts half warm: partial
insertion of the device (position B)
Engine warm ups: during engine
warming up, even if the vehicle is under way,
the starting device must be gradually pushed into the rest
position.
Normal operation: starting device
must be pushed back as soon as the engine
has reached the operating temperature (position C)
Idling Operation and
Progressive Action
The fuel is carried from the bowl (4)
to the calibrated holes of the idling jets (14) through the
ducts (15). Emulsified with the air coming from the ducts
(13) through the ducts (20) and the idling feed holes (18)
adjustable by means of screws (19) the fuel reaches the
ducts (20) the mixture can reach the carburetor throats also
through the progression holes (16)
ACCELERATION
By closing the throttle valves, the
lever (25) , by means of the shaft (27), lifts the piston
(26). the fuel is thus drawn from the bowl (4) into the pump
cylinder through the suction valve (23). By opening the
throttles, the shaft (27) is free and the piston (26) is
pushed down under the action of the spring (28), by means of
the ducts (22) the fuel is injected into the carburetor
throats. The inlet valve (23) is provided with a calibrated
hole which is discharges the excess fuel delivered by the
accelerating pump in to the float bowl.
SETTING THE FLOAT
1 Make sure that the weight of the float is the
correct one (26 grams),
that the float can freely slide on the axis and does
not show any pits.
2 Make sure that the needle valve
(v) is tightly screwed in
its housing and that the pin ball (Sf)
of the dampening device incorporated in the needle (S)
is not jammed.
3 Keep the carburetor cover (C)
in a vertical position as indicated in the above figure,
since the weight of the float (G)
could lower the pin ball (Sf)
fitted on the needle (S)
4 With carburetor cover (C)
in vertical position and float clip (Lc)
in light contact with the pin ball (Sf)
of the needle (S) the
distance of both half-floats from upper surface of
carburetor cover (C) with
gasket (Gz) in place, must
measure 8.5mm.
5 After the leveling has been done, check that the
stroke of the float is 6.5mm.
If necessary adjust the position of the lug (A).
6 In case float (G)
had not been rightly set, rectify the position of float
clip (Lc) till the required
quota is reached, taking care that the clip (Lc)
does not show any pit on the contact surface that could
affect the free sliding of the needle.
7 Fit up the carburetor cover making sure that
float can move with out any hindrance or friction.
NOTE: the operation of leveling of the
float must be carried out whenever it is necessary to
replace the float and needle valve: in this case it is
advisable to replace also the sealing gasket, making sure
that the new needle valve is tightly screwed in its housing.
EXPLODED VIEW
PARTS LOCATION
5. Idle jet
holders 7. Idle air orifices
16. Additional Air Horns
18. Pump inlet valve
29. Idle mixture adjustment
screws 30. Pump jets
34. Pump control rod 39
Spring guides and retainers 42.
Mixture ducts 43. Starting
Jets 45. Starting jet wells
61. Pump deliver vlave
62. Progression holes inspection screw
63. Emulsion tubes complete
with main jets and air corrector jets
64. Air Horns mounting plate.
65. Bowl plate 66. Idle speed adjusting screw
66. Idle speed adjusting screw
67. Throttle control lever
68. Retainer Washers
69. Pump Spring retainer plate
70. Throttle return spring retaining plate
40DCOE3
SETTINGS
FLOAT SETTINGS
Here are the physical
dimensions of
the body of the DCOE carburetor.
All dimensions are in
millimeters
You are
viewing
the carb
from the
manifold
mount
flange
end.