| 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).
|I can not take credit for the bulk of the
wisdom presented in this article. If I could remember where I found the
original article, there would have simply been a link here and not the full
So... since I had to somewhat format and re-write the layout, I will take credit for the page itself and the fact that this information is available to whom ever needs it.
The credit for the original information as presented here, goes to Mike Kimball and John Trindle. I simply added a bit of formatting, inter-article WWW linking, and page layout.
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):
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.
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
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:
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.
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
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
Well, now that you've optimized the
mixture curve over normal driving / racing parameters, you have probably virtually
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.
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.
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.
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