Understanding a Weber Side Draft Carburetor Through a Fictional Supposition.

[cygnusx1]

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.

[madkaw]

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

[madkaw]

Blue,

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.

[240260280z]

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 May 29, 2012 by Blue

[bpilati]

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 June 4, 2012 by bpilati

[Duffman]

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:

Would 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!

Thanks!

Edited December 14, 2016 by duffman

[240260280z]

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

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

http://www.classiczcars.com/forums/attachment.php?attachmentid=49393&d=1321585578

 

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

 

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Edited December 14, 2016 by Blue

[Duffman]

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 December 15, 2016 by duffman

[240260280z]

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:

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.

Edited December 15, 2016 by Blue

[Duffman]

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 December 15, 2016 by duffman

[Duffman]

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.

[Duffman]

Still confused as to why it is running lean on acceleration, if I have so much fuel in bowl!  Weird ...