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Understanding a Weber Side Draft Carburetor Through a Fictional Supposition.

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I am exploring Weber carbs and decided to try to simplify and present their functioning so that others can quickly grasp the basic concepts. To do this, I decided to make a
l path of design that Mr.Weber, may or may not have traveled.

I will provide further details for operation and maintenance in future posts, however this first post is a very simplified introduction.


The Ideal

The simplest carburetor design that comes to mind would be a tube to flow air with a source of gasoline spraying in the middle of the tube. This would provide even distribution and symmetrical mixing at the highest velocity point of the tube.


Approaching the Ideal: The Main Circuit

The Weber carb commonly used on Datsuns is basically just a tube with a fuel nozzle in the mid-stream of the air path.


Unfortunately another part was required.... the Throttle Valve. To control the amount of fuel and air drawn into the engine a throttle valve was added. Its location was chosen to be downstream from the fuel nozzle as the manifold vacuum on the motor side of this valve would otherwise suck the fuel from the fuel nozzle when the throttle valve was closed. The main drawback of the throttle valve location being down stream is that it still blocks the air flow when open and, fuel sprays over it and can also be deposited on it. In addition to a simple tube with a fuel nozzle design, Weber shaped the tube so that it was narrowest where the fuel was drawn into the air stream by using the Venturi principal. This promoted fuel draw and fuel distribution.

It is interesting to note this simple tube and nozzle design is nearly identical to a modern high performance fuel injected individual throttle body (ITB) manifold architecture.

Reality and The First Corrective Actions: Adding the Idle Circuit and the Emulsion Tube

Unfortunately for Mr. Weber, the above design was only marginally optimal for middle to wide open throttle and it failed completely at idle.

Here are the main problems that he faced:

  1. At lowest rpms (Idling) when the throttle valve was nearly closed, there is not enough air flow over the main nozzle to draw fuel. (Lean)
  2. At higher rpms when the throttle valve was moderately to fully opened, the air fuel mixture became too rich as too much fuel was drawn from the main nozzle. (Rich)

Weber was not beaten as the fundamental design worked in principal and could be modified to work. In fact the above design is actually the so-called “main circuit” of a Weber carb.


The way Weber addressed these two problems of fuel flow at both ends of the RPM spectrum was:

  1. To address idling lean, Weber knew that the main nozzle was no where near the high velocity air flow so he added a new and totally separate second so-called “circuit”: the idle circuit. Basically he added a needle valve on the high vacuum downstream side of the throttle valve. This needle valve independently controls the amount of pre-mixed fuel and air (mixed upstream and not shown for simplicity) to flow past the throttle valve and feed the engine when the throttle valve is nearly closed (and the main circuit was not functioning due to low air flow). The simple one circuit carburetor now becomes a two circuit (Main and Idle)
  2. To address the need to lean the mixture in the main circuit as the throttle valve approached "Wide Open Throttle valve" (WOT), he added a “fuel/air mixing tower” with holes in it called an “emulsion tube”. Air and Fuel mixing took place in and around this tower before the fuel was drawn out of the main nozzle. Fuel entered the tower at the bottom of a well and air entered from the top. As the fuel demand increased approaching WOT, the fuel level in the tower& well lowered and in turn exposed more air holes thus mixing more air with the fuel. This ultimately leaned the mixture (as required) in the main circuit as more airflow through the carb occurred.


Emulsion Tube

Weber now had a 2 circuit carb design:

One being the idle circuit and the second being the main circuit…. But how to switch from one to the other slowly and quickly?

Dealing with the Transitions: Adding the Progression Circuit and the Acceleration Circuit.

The Progression Circuit (for slow transitions from Idle to Main):

Weber found that, as the throttle valve slowly opened, when transitioning from the idle circuit to the main circuit, the air flow required to activate the main circuit did not occur quick enough. This delay in the main circuit starting caused a lean spot between idle and main. The solution used by Weber was to simply add more fuel holes near the throttle valve. These holes were drilled upstream of the idle needle valve port. As the throttle valve opened, it exposed each successive hole to the manifold vacuum. In turn, (due to high velocity air and manifold vacuum) it would draw more mixed air/fuel from each progression hole. For economy, the source for air fuel Weber used was the idle circuit.


The Accelerator Pump Circuit (for fast transitions from Idle to Main):

Weber found that if the throttle valve opened quickly (when transitioning from the idle circuit to the main circuit) the progression circuit above did not have enough time to operate as the airflow suddenly was across the whole carb throat rather than up near the roof and progression holes. This sudden bust of air and delay in the main circuit starting caused a lean spot between idle and main. The solution used by Weber was to add more fuel by using a one-shot-squirt-gun-like pump that was activated on a quick throttle opening (hammering the gas pedal). The design also had a clever feature where slow throttle valve openings did not squirt additional fuel (to be shown in more detail in subsequent posts).



- A Weber side draft carb used on many Datsuns has two independent "circuits": The Idle Circuit and Main Circuit).

- A so-called Progression circuit is used to aid in slow rpm transitions from Idle Circuit to Main Circuit.

- A so-called Acceleration circuit is used to aid in fast rpm transitions from Idle Circuit to Main Circuit.









Edited by Blue
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Thanks, I am in the same boat but now I am starting to put it all together in my head so I figured that I would make a tutorial thread.

I am no where near the expert level that I want to be (yet) but I am presently getting hands-on experience and delving into the theory in parallel so that I am not "parroting" too much :)

For the rest of this thread, I'll add similar details for each circuit, add some rebuilding details, then compile some tuning info specific to Z motors.

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Idle Circuit Drawings



Idle Mix Screw


Idle Mix Hole

Note that it is on manifold vacuum side of the throttle valve and is subjected to a large pressure difference that pulls the air/fuel mixture from this hole. Some air also flows by the throttle valve at idle to dilute this mixture.






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Progression holes can be viewed from above by simply removing a threaded brass cap.

This inspection is important for setting the throttle valve position at idle as it should be at or behind the first progression hole.

In this photo,you can see that the throttle valve is blocking the first of the three progression holes. It is a wee bit too far forward as it should only block~ 1/2 or less.





Edited by Blue
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Key points to understand the functioning:

1. The Bleed-Back/Spill jet sits at the bottom of the fuel bowl.

2. When the throttle valve is closed (idle), fuel enters the bleed-back valve, flows past the ball bearing valve and fills the pump chamber as the rod is pushing the piston is up high in the pump chamber and the spring compressed. Weights atop the two other ball bearing valves keep them closed and prevents fuel from flowing out of the two jets.

3. When the throttle is opened quickly, the rod drops and the spring pushes the piston down. This forces fuel backwards. The ball bearing in the spill jet is forced upward and closes this valve preventing fuel from back flowing into the fuel bowl. The pressure also lifts the other two ball bearings and corresponding weights (opening these valves) and allows the squirt of fuel to shoot out each pump jet.

4. The diameter of the pump jet holes, the length of the piston's excursion, and the spring tension affect how much fuel is squirt and the time duration of the squirt.

5. When the throttle is opened slowly, the rod also drops but the fuel squirts slowly back into the fuel bowl as the ball bearing valve in the bleed-back/spill valve is designed to "bleed" on slow throttle transitions. In fact the progression circuit is designed to be the key supplier of fuel during slow throttle transitions...however some fuel will inevitably go through the pump jets. Size of the bleed back valves, accelerator ball bearing weights and piston spring pressure are the key factors in how "leaky" the pump jets are in slow to medium transitions and also can be customized for normal fast transition squirts.


Accelerator pump rods and springs can come in different sizes. A notable Weber user on this site recommends 10mm rod throws for Datsuns.


The pump jet. Note the a very very small washer/gasket accompanies this part and comes with most rebuild kits.


Back-Bleed/Spill Jet that sits at the bottom of the fuel bowl






Edited by Blue
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An amazing job on a Suppositional Understanding of side draft carburettors. Dave(Z'sOnDaBrain) should do some electrical breakdowns like this. It really simplifies and takes the intimidation out of understanding and tuning them.

Keep up the GOOD WORK!

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Thank You Very Much for making this great thread. I have the Weber book, and it confuses me because it covers different variations of the carb, that I don't need to know about. You have made it 150% easier for me to see what each part does and how the whole system functions, and I greatly appreciate it.

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Good explanation. One of the reasons, if not the main reason, they emulsify the fuel with air is to reduce it's density so that it can accelerate through the passages more quickly, increasing the fuels response to demand.

I am about to go pick up a full set of triples with linkage and manifold. I can't wait to rebuild them! It's so relaxing.

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  • 6 months later...

Hey Blue,

When you get time-can you do a thread like this on the Mikuni's!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Very nice work, and I do mean work to put all this together-thanks

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After reading your thread again and again I have new leads for my issues with my Mikuni's . Not sure if Mikunis have an inspection plug for the progression holes, but thats where I need to look next since my engine falls on it's face over 1500 rpm just sitting in the garage. I should have inspected them while I had the carbs off- they maybe clogged.

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I have been meaning to update this article with tuning guidance following further study on a side draft forum but the data I have so far is inconclusive. I have a set of Webers here that I will experiment with once I break in a resto's engine.

One of the problems I found in working with an experimenter on the side draft forum is that the accelerator circuit leaks fuel all the time and it lowers the air:fuel ratio. I speculate that the engine vibration shakes the weights and allows fuel to pass AND there is a vacuum at at accelerator circuit's nozzle at partial throttle as the top of the throttle valve approaches the tail of the main venturi.

This leaky accel. circuit makes tuning more difficult. At present, here is what I would do with Weber DCOE's to tune.

1. plug inlet jet/ball valve leak back to accel. circuit at bottom of fuel bowl.

2. set throttle linkage so that valves are covering the first progression hole.

3. remove main jets.

4. experiment at low rpms and tune the idle circuit for idle and low rpm operation

5. install the mains and tune for high rpm and WOT performance

6. install the accelerator circuit jet/ball valve AND add springs to press the weights down OR replace the weights with springs.

7. adjust the accelerator circuit for slow and fast RPM transitions

You will have to wait until I get Mikunis. My next project will be ITB's and Mega squirt.

FYI here is data from my friend showing the AF ratio and proving a leaking accelerator circuit. (plugged vs non-plugged during slow accelerations (2 min.) to prevent activating the accelerator circuit). This test was repeated 3 times. The red numbers show the change in Air:Fuel Ratio at each corresponding RPM.


Here is where I think the vacuum pulls on the accelerator nozzle and causes the greatest fuel leak at cruise:


Edited by Blue
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Interesting to compare/contrast the Weber design to the SU design with a machined tapered needle to vary the orifice. Both approaches have their merits.

Well the SU is a variable venturi, constant velocity carb. So theoretically the air volumetric flow rate varies proportional to the fuel flow (i.e., tapered metering pin and orifice), while the air velocity remains constant through the range. The intent is to give your smooth power throughout the range. Fixed venturi carbs have multiple jets, mutiple venturis (barrels) to adjust to changing engine requirements. Air velocity changes with demand.

Edited by bpilati
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  • 4 years later...

Blue, this is great stuff, as noted above, but some of your gifs have been lost:

FYI here is data from my friend showing the AF ratio and proving a leaking accelerator circuit. (plugged vs non-plugged during slow accelerations (2 min.) to prevent activating the accelerator circuit). This test was repeated 3 times. The red numbers show the change in Air:Fuel Ratio at each corresponding RPM.


Here is where I think the vacuum pulls on the accelerator nozzle and causes the greatest fuel leak at cruise:


50721d1327113402-accel-vs-choke.gifWould you be able to provide those?  I am new to Webers and interested in the leaking/dripping issue you mentioned above, would love to see the test results!


Edited by duffman
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Hi Duffman,

No problem. My buddy Ross Parks is in your neck of the woods this winter. Say hi if you see him.

The biggest source of leaks with Weber DCOE's are:

1. Fuel Level Set Too High. (27mm to 30mm down is where you want to be)

2. Aux Venturi sealing inside the throat


Some Passini for you too.  As mentioned in another thread some good Weber info will be shared in a month or two :)




Look at the pump leaking at the phillips screw in this video at high rpms.  I speculated and discovered then later saw that Passini knew of it all along :) Nothing new under the sun.

Fuel _PullOver_ from accelerator pump squirters..avi



accel leak.gifaccel vs choke.gif

Edited by Blue
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Thanks, Blue, I have much to learn.  My 40's are dripping off the pump jet in a number of the chambers, part due to the pump jet cover not being screwed down tight enough, but I need to confirm the fuel level to make sure this is not contributing to the problem.  I am getting fuel dripping off the front of my air horns, in some cases.  Also, goes very lean on hard acceleration, then richens as the main jets kick in.

My specs: triple 40 DCOE carbs, 30 mm venturis, auxiliary 4.5, idle jets 50F9, main jet 125, air corrector 175, pump jet 45, pump exhaust valve 050, emulsion tube F11, float needle valve 1.75.  Idles like a champ at 750, hard acceleration runs 15.5 to 16.5 on AFM, flutters and farts on deceleration (super lean), typical transition miss from idle to mains. 

Once I confirm (and correct, if needed) fuel level, will look at pump jet size and acceleeration pump performance to see if I am missing something there.

Will try some of your ideas once I get the fuel level confirmed!  Open to any other ideas or thoughts!

Edited by duffman
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I found 55F9 Idle jet worked nice on a 240z.  It will give you more fuel in the transition and you can dial it back at idle.

The lean on hard acceleration seems strange with a 125 main.  I wonder if your fuel level is too low?  What version of 40DCOE do you have?

For the leak, it could be very small aluminum washers that may be missing on the pump jets:

pump jet.jpg

Or the aux venturis may not be sealing well against the throat. This could explain the leak and weakness on acceleration.


For completeness the choke plungers can leak. If you have them in your carbs you may want to disable the choke cables then ensure the plungers are all the way down.


Here is how to check for correct fuel level.

29mm Fuel Level.jpg

Edited by Blue
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Good stuff, Blue!  My carbs are the 151 version of 40 DCOEs, on a stock, rebuilt L28. 

For clarification, dripping is not out the air horns, but underneath the horns, can feel some fuel on the bolts underneath the carbs that secure the air horns to carb.  Have doubled up the mini crimp washers on the pump jet and, along with tightening them down further, this has stopped the dripping out of most of the barrels.  Venturis seem to be seated correctly, will double check.  Choke cables are not connected, don't need them in AZ!

Appreciate your comments on the idle jet size (will play with that) and how to measure the correct fuel level,  Will report back with results. 

Thanks, Duffman 

Edited by duffman
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Measured my float, per instructions, at 14 mm down from lid.  Measured the level of fuel in aux venturi tunnel, and it is only 16.25 mm down.  Can't believe it would be off so much!  All 3 carbs measure the same.  I used the zip tie approach, marked it at 29.5 mm and stuck it down the tube.  I measured from the top, where the tube actually screws in, not the top of the carb lid.

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