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How To Use a Wideband To Tune Your Carburetor(s) On The VW Flat-4 Engine

Tuning with a Wideband enables you to seriously dial in your carburetion system. This article contains both the “how” and the “why”. It is important that you understand the “why” because when you make a change, observing how the engine reacts to the change will tell you if the change was a good one, didn’t matter, or you went the wrong way!

I urge you to read this article at least 2x, with at least a few hours in between to reflect upon and digest the content. You will likely get a lot more information the 2nd pass, and even more if you read it a third time! And, as the title of this article suggest, you are also going to need to get a wideband air/fuel (A/F) gauge. We recommend one of these two:

Innovate LM-2 Toolbox Wideband Kit   or    Innovate MTX-L Dash Gauge Wideband Kit

A Wideband allows you to read the exhaust gasses post-combustion. Once you know how much oxygen is remaining in the exhaust, from that you can deduce the A/F of the engine. HOWEVER, the gauge can (WILL) read wrong if you have exhaust leaks before the O2 sensor. With a leak present, the exhaust will suck in air, and give the O2 sensor a false lean reading. The gauge will also read wrong if you have a mechanical issue with the engine. Examples are a vacuum leak allowing unmetered air into the engine, running lean. Another example is a leaky valve (burned valve, poor valve job, or tight valve adjustment causing a leak. If these happen you have an entire cylinder with low or no compression, which won’t combust, and the unburned O2 makes it’s way to the exhaust (reading lean on the wideband). The last very common problem is an ignition problem (ignition miss), which will also show on the wideband as “lean”, as all that O2 goes into the exhaust system. It is critical that you understand that the wideband reads OXYGEN, not fuel!

An engine will run pretty easily, even in poor tune. Low compression, lean or rich mixture, retarded or overly advanced spark, etc. To get “good combustion”, all need to be within a tight range of operation. This creates a smooth and efficient engine, which doesn’t hesitate, and doesn’t create too much heat. This is achieved when there is good vaporization of the air / fuel mixture, as well as an ideal ignition advance point (spark at the spark plug).

Other VW Carburetor Tech Articles


First, we have to go into the theory of combustion in the engine, and how important good fuel vaporization is. The engine will, in fact, run if you pour gasoline down the air cleaner throat, but this is not good vaporization. Fuel Injection gets the fuel into a nice mist, but note this is still not well vaporized. Carburetors are primitive devices, but they do accomplish their task quite well if they are sized and tuned properly. We are going to go into how a carburetor works in another article. THIS article will be focused on how to tune the carburetor(s) you have. So let’s get started!

First, you need to know how to set ignition timing. Also set and check fuel pressure and float levels. The carburetor(s) will not work properly without these being in an operational range! Check your specs, and make sure it’s set. Don’t assume someone else did it, YOU DO IT.


The carburetor float levels must remain constant so that the main circuits always operate under the same conditions. Set your carburetor float levels before yo begin! I can’t tell you how many times I have had a customer making jet changes for weeks, only to figure out he had the float set improperly the whole time, and had to re-do everything after he figured it out!

Back to tuning, there are 2 different ways to read your wideband gauge.

Lambda Method “λ”

Air / Fuel (A/F) Method (ie: 14:1, 13:1, 16:1).

A/F is the ratio of air to fuel, air/fuel, by mass. Different types of fuel have different ratios that they burn (oxidize) at. Chemically, gasoline works best at 14.7:1 (14.7 parts air to 1 part fuel). Note that this is a CHEMICAL optimization. In an engine however we don’t have a lot of time to get every molecule of fuel to find every molecule of O2 for a “perfect” mixture ratio. So we run “richer” than this to get the most power. By running a little richer we use all the O2 up, and maximize our power.

In the Lambda Method, the ratio is simply compared to the chemically perfect ratio, for the fuel you are using, where gasoline (14.7:1 A/F = 1λ).

AFR / λ conversion :

Since the values ​​of λ or AFR will be used in the lines that follow, here is simply how to convert a value λ to an AFR.

Simply multiply the value of λ by the stoichiometric ratio of the fuel concerned.

If the value λ is equal to 1 you are in the stoichiometric ratio of the fuel, a value λ > 1 indicates a leaner mixture, and a value λ < 1 is indicates a richer mixture.

Note that different types of fuels have different optimal chemical ratios. Ethanol requires more fuel than gasoline for a chemical ratio match. Same with methanol, or nitro. Don’t get too hung up on the #s, just know that your gauge uses Lambda, and CONVERTS the display to A/F for you. The wideband doesn’t know what kind of fuel you are using, it only reads O2 (Oxygen). If it reads .85 λ, this is .85*14.7:1 = A/F of 12.5:1. 1 λ = 14.7:1. 1.15 λ = 16.9:1. I recommend you pick one method to read, and stick to it exclusively. The λ Lambda Method is less prone to conversion errors, and works regardless of the fuel you are using.

Most folks don’t know that in original form (IE: Stock), our engines were often tuned in the 11.5:-12:1 A/F (λ .80) range. This is quite rich. This is because back when the cars were first made, they didn’t care at all about emissions. Cars were relatively primitive, and just running was a miracle as it was!

For the scope of your tuning, there are basically 2 or 3 windows you want will be operating in (your A/F mixture).

Idle = 13.5 – 14.7:1 (λ 1.09 – 1)

WOT (Wide Open Throttle, or anything over 1/3 throttle) = 12.8 – 13.2:1 (λ .87 – .9)

Light Cruise with vacuum advance = 15.5 – 17:1 (λ 1.05 – 1.15)

“Light Cruise” is throttle settings under 1/3 of full, but not idle. Generally 3-4k RPM. Light Cruise is NOT used for engines which do not have any vacuum advance compensation. For our engines, if the engine has a 1971 and newer distributor you CAN (but don’t have to) tune for Light Cruise. In most cases I recommend simply tuning for idle and WOT conditions initially, and then chasing the Light Cruise stuff later on after you get a grip on what is going on. Light Cruise tuning can be a huge headache for folks, since you can get lean mis-fires which you know throws off the wideband. Generally you will immediately see this as a spike A/F reading of 20-22:1 (λ 1.36+).

To further complicate using a wideband, picture this scenario. You have a mechanically sound engine (good compression), and the ignition is working perfectly. Your spark plug is reading very lean, while your wideband is showing rich. Huh? How in the world can the gauge be 11:1 yet the plug is white? Simply, this is an example of poor combustion, and I will explain.



This situation like this is not uncommon, especially on our primitive engines, where the fuel is is not vaporized properly, and the result is that THE COMBUSTION IS COMPLETED IN THE EXHAUST RATHER THAN IN THE ENGINE.

The spark plug reads lean while the A/F reads rich. The poor guy trying to tune the engine sees the rich AF on the wideband, and adds more fuel, only to see the problem worsen. Talk about driving him crazy! It’s not uncommon for guys just beginning to tune with a wideband to see A/F readings of 11.5-12:1(<λ .8).  This can cause a problem with excess fuel washing the oil of the rings and cylinders, and the engine loses ring seal or wears out very quickly!

This is an example of where the engine is seeing a different A/F than the wideband. But why is the plug reading so vastly different than the wideband?


First, the spark plug color is shown by the mixture in the cylinder. A wideband lambda sensor reads the total amount of oxygen in the exhaust, but it does not tell you about WHEN it combusted (Burned). Basically, we have a situation where the combustion actually completed in the exhaust system, but before the O2 sensor. This is either extremely late ignition timing, or poor vaporization of the fuel. I have observed this situation a lot. But how does it happen?

Basically, a lot of VWs do not have enough heat or energy to vaporize the fuel. We basically have a fixed amount of heat in the carburetor, intake manifold(s), cylinder, head, and heat from compression, being used to vaporize “too much” fuel. Proper pre-combustion temperature is what will properly vaporize gasoline which is properly metered (not poured out of a hose). carbs_c.jpg If the engine is too rich, we can have this situation. But if we lean it out, then the fixed amount of intake heat available will be absorbed by a smaller volume of gasoline, and this results in better vaporization of the fuel (and a better mixture). This improved mixture will color the spark plug properly, and the plug will color indicative that the wideband reads. Re-phrased, the amount of heat available to vaporize gasoline is constant (low), we must simply reduce the amount of gasoline to have a complete combustion. We want to COMPLETELY vaporize our gasoline (not just meter it), yet many people ride rich to “run cooler”, which kills engine performance without even realizing it.

What you want to avoid :

Many VWs run at 11.5: 1 AFR (λ .80) and even richer. When you are this rich, the average combustion temperature is “cold”. These engines are often identified by having a LOT of ignition advance. See “cold” mixtures have slow combustion. In Chemistry class you may have learned that the speed of chemical processes doubles/halves every 10 degrees Kelvin. A lot of VW owners overdo the “keeping my engine cool” thing with oil coolers (fans at 180F0, and no thermostat nor flaps in the fan shroud.

An engine tuned rich and with a lot of advance seems to run well because putting a lot of ignition advance in, the A/F mixture has TIME to absorb heat in the combustion chamber before the spark plug lights it off. The engine runs, but it’s not ideal.

So what is a better approach in order to have a fast combustion speed? Really it’s the combination of ignition advance AND fuel mixture working together. A change in one usually requires a change in the other, to be optimized. A lot of guys jet the carburetor(s) to match the ignition, rather than realizing they work in concert with each other. If the mixture was leaner, it would be ready to combust earlier, while still not being close to detonation. This engine would be much more efficient if its mixture was leaner and had less ignition advance, rather than being pig rich and advanced like crazy.

An example; if you set your ignition advance at 32-34° max. With so much advance, you HAVE to enrich your mix to get the best out of your engine.

carbs_d.jpgMost people adjust their ignition to 30-32 °, then adjust the maximum acceleration to have the greatest power. All they did was find the best AFR for this value in advance. In fact, most engines will be more powerful and cleaner if the ignition advance were less and they were running a leaner A/F. Note that “leaner” does NOT mean “lean”. Ideal WOT A/F is 12.8 – 13.2:1 (λ .87 – .9).

Most people set things so rich that they must put a lot of ignition advance, since the combustion occurs so slowly because of the cold mixture (too rich). They simply do not grasp that they would need less fuel if they put less timing in. Most folks believe that “all engines are more powerful with more advance”.

This can be easily demonstrated on your engine. If you ride extremely rich (like 10: 1), you will need to put a lot more ignition timing(10° more) to run as it did before, and everything would seem fine. But it’s a lot richer. Conversely, you can try to take out fuel and take out timing too. What does the wideband say?

If you can convince yourself to try this, often times your engine will wake up!

B)Idle speed adjustment method :

In order to properly adjust your engine, set the ignition timing to idle around 8 ° before top dead center. Then set the idle speed to 800 RPM, synchronize the two carburetors, adjust the idle speed (refer to the paragraph on adjusting the idle richness).

This is a temporary setting that will serve as a basis for work.

Now you have to set the throttle plates to a good position so as not to uncover the progression holes. If you have a vacuum plug for the SVDA port on the carburetor, remove it and connect a hose and listen. Or connect a vacuum gauge.

Tighten the idle speed adjusting screw (idle speed increases) until you read a reading on the vacuum gauge (or until you hear a suction sound at the end of the hose). Now loosen until the vacuum disappears.

Synchronize the second carburetor.



From now on, any change in the idle speed setting will now be done with idle bypass screws or ignition timing.

Check that the two bodies on the same carburetor have the same depression, if it is different, you may have a twisted throttle pin.

Set the idle mixture again (Lean Best Idle + ½ turn richer (OUT).


Idle mixture is of course set engine warm, as the name suggests, it is only used for the idle speed, it does not affect or very little rich mixture on the carburetor progression circuit.

Note we do NOT want to set idle mixture for highest RPMs. Chemically the highest RPMs will be at “stoic”, but this is also unstable. 1/2 turn richer will be stable and still pretty lean. Setting idle mixture to stoic (highest RPMs) will result in an unstable engine, the engine idle speed can drop if you just turn the headlights on, or the idle speed will change as the temperature does. It will drive you crazy. On an engine with no choke, if you simply tune to 1/2 turn richer than “stoic” (fastest idle) the engine will be much happier and more predictable. A properly set idle mixture will likely pop and fart a little bit when dead cold, this goes away after 30 seconds or so of running. We want to get a steady idle speed in all conditions (hot / cold, dry / wet) and to achieve this, we simply adjust the idle mixture a little richer than “ideal”.

How? Begin by loosening the idle speed screws slightly, then on the first cylinder screw it in (leaner) until you hear the engine “die” on the cylinder. Then loosen it again until the cylinder wakes up. Continue to loosen (richer) until idle speed is at its peak. Be sure to wait 20-30 seconds after turning the screw for the rpm to stabilize! Rushing is a rookie mistake! Proceed in 1/2 turn increments. If you have one of the old school garage tachometers which lets you see RPMs every 10, that is ideal! But really you can hear it if you listen closely. Find the point of the fastest idle, Once you have found this highest speed, loosen the Idle Speed ​​Screw ½ turn again. Do the same on the other 3 cylinders. Now you have to go back and re-do all 4 again. And then a 3rd time. By the 3rd pass you probably won’t be actually changing anything, you are just confirming it.

This method will provide a more stable idle in all weather conditions.

Note that any time you change one of these three:

  • idle jet
  • idle speed
  • ignition timing

you must re-adjust the idle speed setting all over again, since the engine conditions affecting idle have changed (and the ideal idle mixture setting is now different than before).

After adjusting the idle mixtures, what is your idle speed? If it seems too high, remove a little idle advance (ignition) and set the idle mixture again.

If your idle speed is now too low, you have two options:

If your ignition advance is around 8° before TDC you can try to open your idle air bypass screws. Do this on all the bodies so that they have the same reading on the snail gauge. You have only a small adjustment range with these screws. If you still do not get the desired idle speed, you’ll be adding more ignition timing which will increase the idle speed. Simply add in idle timing until you reach the desired idle speed. I do not recommend an advance greater than 12 ° before TDC at idle. If you need so much in advance you may be something that is wrong. Like 20-50 oil which is so thick it’s slowing the idle speed. I have seen a 300RPM increase just with an oil change to a thinner oil. Obviously this requires re-setting the idle speed and idle mixture screws due to a huge change in vacuum.

For center mount carburettors, they need more air to increase the idle speed. On these you get the desired rpm the same way, but something have to also drill the throttle plate with a small hole to increase the air volume.

Be aware that the optimum advance setting at idle speed has nothing to do with any A/F readings in a non-idle situation. So if you use a non-adjustable distributor, you may have a situation where the idle speed setting determines the maximum feed value and vice versa. If you want a maximum advance of 28 ° and you adjust the idle from 12 to 8 °, the range of advance of your igniter will have to be changed in order to keep the 28 ° of advance max. You simply have to open the distributor up and manually adjust the stops, to get the centrifugal advance RANGE needed for your engine. I am not going into that in this article.

By doing all this work you will permit the carburetors to function as they were designed to. It is critically important to have the butterflies properly positioned just below the progression holes, I can not stress enough. This is why it is important to use your carburetor’s SVDA port, even if you do not use a distributor with a vacuum line) to determine the position of the throttle plates at idle. If the throttle plates are too open (vac at idle) setting idle mixture is difficult, since the engine is also getting fuel from the progression holes, and the idle mixture screw effect can be insignificant. If the plates are not open enough you can get a hesitation off-idle.

Next we move onto the actual jetting changes!


The sizing of the jets is done in this specific order for the best results. You’ll wind up with good MPG, cool running, and maximum power at full throttle.

We jet the carburetor(s) in this order:

a) Idle Jet Sizing.

b) Main Air Jet Sizing.

c) Main Fuel Jet Sizing.

These tests will require the use of a wideband AFR device. You’ll do these on the road, or the dyno. I strongly suggest disabling the pump jets so they don’t throw you off with extra fuel. We’ll adjust those at the very end, needing as little as possible.

How does the progression circuit work?

The progression circuit of a carburettor comes into action depending on the position of the butterflies. When the butterflies are open less than 1/3 opening, engine vacuum in the intake pipe is very strong at the entrance of the progress holes machined in the body of the carburetor. This vacuum pulls air/fuel mixture through these holes. The progression circuit is necessary because the low airflow (low throttle settings) isn’t enough to operate the main circuit of the carburetor. You can see this by dismantling the air nozzle assembly, emulsion tube, main jet and drive the progress circuit to see which accelerator stroke it is running.

Really your carburetor is 2 carburetors. 1 for idle and low throttle settings, operating through the idle mixture screw and progression holes. The other is the main circuit, operating through the aux venturi, emulsion tube, main jet and air jet. For now, let’s focus on the idle jet.

Idle Jet Selection (requires a wideband) :

A common mistake is to adjust the idle jet size according to the value given by our AFR at idle speed. The reality is that we can run a richer idle with a 50 idle jet than with a 80 idle jet, depending on the adjustment of the idle mixture screw. The idle mixture screw is what sets the idle mixture! The sizing of the idle jet should be based on the engine behavior on the PROGRESSION CIRCUIT. We can set the idle mixture later with the idle mixture screws, regardless of the idle jet used.

So choose the idle jet according to how the engine runs on the progression circuit. To do this, you have to know EXACTLY where the progression circuit is located. We do this by disabling the main circuit of the carburetors. Simply remove the air jet / emulsion tubes / main jet assemblies from the carburetors, and store them in a safe place.

Without these jet stacks, a HUGE LEAN HOLE will appear as you leave the progression circuit, since there is plenty of air but not enough fuel.


I recommend a few HOURS (not minutes) of driving without the main stacks in, so you get very familiar with the operation and location of the progression circuit of your carburetors.

The way the progression circuit operates is not adjustable, so the richness on this circuit is adjusted only by the size of the idle jet. Finally, remember that every time you change the idle jet size or the idle speed, you will need to readjust the idle mixture to get back where you were before.

Now that you have the air jet / emulsion tubes / main jets removed, we can work on the progression circuit. We will simply look for the idle jet size to read a A/F value we want. For now let’s shoot for 13:1. Really 12.8-13.2:1 is ok. Avoid the stoichiometric zone around 14-15:1 at all costs, this is where your engine will be the hottest. Make sure you change idle jets in increments no greater than 2.5. It is a common mistake to change by 5-10 at a time, which is a change of 20-40% of fuel! From too lean to too rich, or too rich to too lean if you change by 5 or more.

Remember, carburetors are relatively simply (primitive) devices, and they are NOT perfect. It is normal to get a swing of .5:1 A/F, even in a perfectly jetted carburetor. When you are satisfied that you have done what you can do with the idle and progression circuit (idle jet sizing), you can move on to the main circuit. Remember, this behaves as a totally independent carburetor circuit, in addition to the existing idle and progression one you just finished up with.

Put the main stacks back into the carburetor. Usually the main circuit won’t be perfect, it will come in at the wrong time, and overall fuel metering will be off. We have to adjust WHEN it comes in first, so that it comes in when the progression circuit is completed. You know that spot because you just spend hours (weeks) driving the engine with the main circuit deactivated, RIGHT?

Ideally you’ll see the wideband continue 13:1 from progression to main, as you gently increase throttle. But normally you’ll either see it swing rich for a bit, or lean for a bit. Rich means you are still in progression while also metering from main, and lean means the progression has completed yet the main hasn’t kicked on yet. I call when the main circuit comes in as “tip in”. You can adjust this a little bit, and you do this via changes in the emulsion tube, venturi, or air jet. I am assuming you have the venturi and emulsion tubes properly selected.

Emulsion tubes?


The emulsion tubes have major effects on the tip in action of the main circuit, and it is already known which ones work best on our Flat-4 Aircooled engines. Run F11s for 2bbl Webers up to 38mm venturis. F2s for Weber 2bbls 40mm venturis and larger. F7s for race engines. Dellorto dual DRLAs run the .2 emulsion tubes You can e-mail me for advice on these items, I am trying to keep this complex topic on point! Know that once you get close on the venturi size, and if the emulsion tube is correct, you will be able to tailor the tip in simply with the air jet sizing.

Air jets :


The air jets have effects on the tip in of the main circuit. We use them for a fine adjustment of the transition from progression to main circuit.

Generally, a small air jet will delay the start of the main circuit, while a large air jet will activate the main circuit sooner.

We simply adjust the size of the air jet so that the transition between the progression circuit and the main circuit is as seamless as possible. Once this size is set we will no longer feel this transition. I remind you that the accelerator pumps are always deactivated.

So mount oversize (160) main jets and start with relatively small air nozzles (150-160). The goal will be with the help of the wideband to gradually increase the air jet until the lean hole during transition between the progression circuit and the main circuit disappears.

Note that this main jet is actually way too big, but this makes it easier to see on the wideband when the main circuit actually goes into action. This exaggeration lets us play with the air jet sizing to get the timing correct, then afterward we play with the size of the main jet to get to volume right!

Start with a small air jet and a large main jet, and you should have a hole (poor). Increase the size of the air nozzle so that the hole decreases until it disappears. When the main circuit comes into action, your AFR mano will switch to the rich, very rich (~ 12: 1, the exact value is not important) given the exaggerated size of the main jets, which is important is that you no longer have the poor mixture of the progression circuit (16/17: 1). Once you’re done with this hole, you’re done with the air nozzle setting.

We can hit the sweet spot by tuning from the rich side leaning as we go, OR by tuning lean and richen as we go. I choose the latter, since it is much easier to feel a lean hole than a rich hole. We are simply looking for a reasonably smooth transition (minimal), without crazy rich/lean swings on the gauge.

GENERALLY SPEAKING, as you increase the air jet, the main circuit will come in sooner. If air jet changes aren’t doing this, then you likely have something else going on (ie: wrong size venturi, emulsion tube, fuel pressure, float level, etc). I like to put a big main jet in, and big air jet. The big main makes it so the gauge goes WAY rich when it turns on, there is no question when the mains have tipped in!j Then simply drop the air jet down until you get the lean hole (progression off, main off). Once you do, you have gone too far. Simply go back up on the air jet to get the transition back. Then size the main jet to get your 13:1 A/F for most main circuit operation.

When tuning, a common mistake people make when experiencing a lean hole (progression off, main off) is to increase the size of the idle jet to add more fuel, covering up the hole. Richening the progression circuit will actually extend the range of carburetor progression, but it richens up the ENTIRE progression to simply fill a small trailing hole. This is wasteful. Why run rich from idle to at least 2500 RPM to fill a small hole from 2500-3000, when you can fill just this small hole if you know how? Most don’t know, but here’s the trick!

Simply get the main circuit to tip in SOONER. So how to do that? I typically increase air jets 15-20 at a time, until the hole is covered. If you go too far the A/F will swing rich, since you are metering progression AND main at the same time (main tip in too early)

This is where experience helps. Or at least having a lot of main and air jets to experiment with, and to experience first hand what it’s like for the change and affect! Experience is the best teacher!

We want the main circuit to take over the progression circuit. Once the air nozzle size is set, the AFR should now go from 16: 1 (progression) to ~ 13: 1 (AFR searched for the main circuit) in a fast way, avoiding the area as much as possible. stoichiometric (14.7: 1 for gasoline).

What happens if the air jet is too big ?

Remember that if you start your adjustments with a big air jet, the main circuit will come into action too early and will be superimposed on the progression circuit. You’ll be way too rich before the progress circuit effect fades away because you’re bringing in fuel from the progress circuit and the main circuit. If the air sprinklers are too big the AFR will switch to very rich (less than 12: 1) since the main and progress circuits work together, then get poorer as the progress circuit effect fades.

What happens when the air nozzle is too small ?

If the air nozzle is too small the AFR mano will immediately switch to a very poor value (20: 1) since the effect of the progress circuit fades and the main circuit is not yet in action. .

That’s why it’s very important to know how your progress circuit works by testing without an air nozzle / emulsion tube / main jet when adjusting the idle jet size you’ve been able to lead before.

The progress circuit is intended for the road on a gas net (poor AFR), while the main circuit is made for use in charge (AFR for power not for consumption). Do not confuse them and the air nozzle will allow a good transition between the two.

If despite all you have problems getting the main circuit in action at the right time by changing the size of the air sprinkler you will need to change the emulsion tubes. You may also have to change the tank level, but the levels of tanks and emulsifiers that work on the Flat4 are known according to the carburetor model. Note that the tank level of a carburetor is not adjusted out of the box.

What are we looking for ?

It is necessary that this superposition between the circuit of progression and the main circuit is minimal, that is why we will start the adjustment with a small air jet to have a hole well present at the beginning of the setting that the we will disappear as we will increase the size of the air nozzle.

H) The main circuit of the carburettor:

The carburetor main circuit is used to take over the progress circuit when the carburetor throttle is open. Since now the butterfly is open, the depression in the intake pipe is now low which makes the progress circuit inoperative (more depression in the progression holes). On the other hand, the increase in the speed of the air entering the carburettor and further accelerating through the nozzle will initiate a depression at the level of the diffuser sucking through it an air / fuel mixture dosed by the main jet , the air nozzle, all mixed by the emulsion tube.

Adjusting the size of the main jet :


Now that the size of the idle and air nozzles has now been selected, setting the size of the main jet is very simple. It consists, assisted by our broadband AFR mano to look for a stabilized value of wealth around 12.5 / 12.75: 1 during a full load acceleration. The ideal is to run at a steady speed by applying a progressive load on the engine to avoid a possible hole caused by the temporary absence of a return pump until a full gas acceleration. When you are full throttle watch your wideband AFR mano and play on the main jet size until you hit an AFR between 12.5: 1 and 13: 1, ideally 12.75: 1.


If the AFR is greater than 12.75: 1, for example 13.5: 1 (a little too poor for a full throttle acceleration), increase the size of the main jet, if the AFR is less than 12.75: 1, for example 11.5: 1, decrease the size of the main jet.

The recovery pump :

During a sudden acceleration, the throttle is completely opened, which causes a rapid increase in the air flow, which is not followed by an increase in the fuel flow due to the greater inertia of the fuel. this last.



To avoid a “hole”, that is to say a sudden drop in speed, we use the action of the pump recovery. This sends during the recovery an additional amount of fuel. On a diaphragm pump, the closure of the butterfly relaxes the return spring of the diaphragm and the diaphragm, by withdrawing, causes a depression in the pump chamber. The outlet valve prevents fuel from escaping as the inlet valve rises, allowing sufficient fuel flow to quickly fill the pump chamber.

The amplitude of the stroke of the diaphragm determines the amount of gasoline injected, while the width of the outlet orifice defines the output speed of the fuel pumped.


Adjusting the return pump screws :

The adjustment of the quantity of fuel injected by the repacking pumps is the last in the carburettor adjustment. When the rest of the carburation is correctly adjusted, it is not necessary that they inject a large quantity of gasoline. Screw the return pump rod nuts just enough so that the slight hesitation that may remain in case of sudden acceleration disappears.

In general, the flow rate of the necessary recovery pumps is much less important once the good transition is found between the progress circuit and the main circuit.

What not to do :

Most people adjust their resume pumps at the beginning to make driving easier, but their action will distort the AFR reading because of the consequent enrichment of the mix they provide. It is therefore necessary to adjust them in LAST when all the rest of the carburation is correctly set.




The carburetion adjustment is carried out as follows:

1. Adjust the position of idle throttle just below the machined progress holes in the body (using a vacuum hose).

2. Synchronize the second carburetor.

3. Adjusting the idle speed with the idle screws to a value at the bottom of the 14 / 14.7: 1 range a little richer than the stoichiometric point for good idle stability. The idle speed should be around 800 RPM.

4. Adjust the idle speed to the desired value by adjusting the idle by-pass screws, or by increasing the idling advance value, check that the maximum advance does not exceed 28 / 30 ° before TDC.

5. Idle jet selection, main nozzle assemblies / emulsion tubes / air nozzles removed. We aim for an AFR of 16/17 in order to obtain a low consumption in cruising. Re-adjust the idle speed at each idle jet change.

6. Selection of the air nozzle, main jet assemblies / emulsion tubes / return air nozzles, with oversized main nozzles. The hole between the progression circuit and the main circuit must disappear by gradually increasing the size of the air nozzle.

7. Select the size of the main jet by accelerating at full load, aiming at an AFR of between 12.5: 1 and 13: 1, ideally 12.75: 1.

8. Reassemble the pump stem nuts and screw them in until the slightest hesitation during a very sudden acceleration disappears completely.


Results obtained :

To give you an idea, a guinea pig was driving with a 2110cc with the following config:

2110, engle 120, rocker arms 1.25, cylinder heads 40 * 35, dellorto 40, nozzles of 34, igniter 010 to 29 ° before TDC.

This member began his adjustments with the basic nozzles recommended by the Dellorto Tech Book for the use of 34 mm nozzles, 60 idle sprinklers, 140 main nozzles, 180 air nozzles.

After arranging a broadband AFR mano he made the adjustment as advised by the method we describe here


It finished with 50-degree idle sprinklers, 152 head sprinklers and 190 air sprinklers.

“The engine has never been so smooth, while offering better performance when accelerating to the floor, the 0 to 150 km / h has never been so fast and consumption has skyrocketed. The settings were made at a temperature of 15 ° C, and after the foot-to-floor runs to select the size of the main jet, the oil temperature was 90 ° C and the cylinder head temperature never exceeded 175 ° C. ° C. ”

Another guinea pig obtained from his pair of Dellortos a consumption of 7.8L / 100 km on his bay window of 72, all in a creaminess never felt before.

So convinced?

At your settings and thanks to John Connolly @ Aircooled.Net!


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