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Re: Weber 34 ADF Tuning

Posted: Sun Nov 26, 2017 10:30 pm
by bartigue
The primary idle jet determines how much fuel flows to the mixture screw; the mixture screw is used to set the amount of this fuel delivered at idle speed. I think an explanation is warranted on exactly how an idle jet works, since an absolutely unnecessary debate has started on the topic, Carl, Tacos, and Beer:

You can verify all of this from the Weber handbook on the 32ADF: https://www.mirafiori.com/forum/viewtop ... 69&t=96376

Idle operation, summarized: Fuel from the float bowl is drawn through a passageway, infused with air, drawn through the idle jet, down a vertical passageway, and through the mixture screw.

Progression operation, summarized: The increasing angle of the primary throttle progressively shuts off fuel delivery through the mixture screw and begins fuel delivery through progression holes (metered by the idle jet) and finally the primary main jet. When the primary throttle reaches about 3/4 open the secondary throttle begins to open, causing fuel to deliver through progression holes (metered by the secondary jet) and finally, the secondary main jet.

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Here is the actual text from Weber:

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Other fun facts:
  • If the primary idle jet is too small for the motor then the car may run with the screw turned out from fully seated, but continuing to turn it makes no difference at all in operation, it neither speeds up nor slows down
  • If the primary idle jet is correct then the car will run with the screw turned out from fully seated and will increase in speed as the screw is extracted further and decrease as the screw is inserted
  • If the primary idle jet is too large then the car will run rich with the screw at any position.
A few misconceptions:

"Jet" is really not a good name, as the "jet" is really just a calibrated orifice and isn't exposed to the air stream at all. If you look at an idle jet it has a side hole and a nose hole. Fuel passes through the nose through the side and onto the chamber. All jets - main, idle, air - calibrate fuel or air in passageways, the passageways feed either the mixture screw orifice (primary side) or progression holes drilled into the side of the carb.

Idle mixture is not set with the idle speed screw(s) and rarely are they set correctly. A car without air leaks and a good clean carb car should idle - roughly - on the mixture screw alone at about 700 - 750 RPM, the idle speed screw brings it up to 850-900 RPM.

If you've ever had a FIAT with a failed idle cut off switch then you know exactly what this means - the car runs just fine except at idle because there is nothing feeding fuel to the motor with the primary idle jet shut off.

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 8:02 am
by friedman
Absolutely correct. I'll clarify my point, You can stick any size idle jet you want in a carb and get good idle because you use the mixture screw to set the proper idle mixture. The fatter the idle jet the more you will tweak the idle mixture closed. The proper sized idle jet (probably a miss-nomer) is selected for transition function.

Anyone who has played with dual IDFs on their spiders knows those carbs are notorious for getting the idle jets plugged with dirt. Nothing like driving a car that wails on good throttle but becomes a three cylinder at idle and low rpm because you are now dealing with four idle jets.

I'll pay for the tacos and beer (or iced tea).

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 9:42 am
by bartigue
Works the same on an IDF, the idle jet size determines the volume of fuel delivered to the mixture jet and the idle progression holes. Anyway, why would you want to set an idle with a too small or too large jet to begin with? Off idle you'll just run lean or rich, then have to re-tune the whole thing. File this under the "you could, but why?" section.

IDFs are a bad example to give anyway; they're not a particularly good carburetor for street use, as noted, they're excellent dirt collectors.

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 12:10 pm
by atruscott
<bearpoke>
How many cars with IDFs or ADFs are you running right now Brad?
</bearpoke>

;)

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 12:18 pm
by friedman
Brad just has to have the last word, even when everyone is agreeing.....it's part of his charm.

The only reason we run IDFs if for aesthetics and aural satisfaction.....IDFs speak to the heart.....I even had them on my 128. I'm not enamored with twin DCNFs but the DCOEs I have strapped to my X are real beauts and they hurt my ears....in a good way.

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 12:24 pm
by atruscott
Honestly... I find IDFs pretty tractable.

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 12:25 pm
by bartigue
friedman wrote:
Mon Nov 27, 2017 12:18 pm
Brad just has to have the last word, even when everyone is agreeing.....it's part of his charm.
I have (and had) no intention of starting an argument, if I did, apologies.

Re: Weber 34 ADF Tuning

Posted: Mon Nov 27, 2017 12:32 pm
by bartigue
atruscott wrote:
Mon Nov 27, 2017 12:10 pm
<bearpoke>
How many cars with IDFs or ADFs are you running right now Brad?
</bearpoke>

;)
Zero! All fuel injected. Well, almost, I have one non-FIAT with carburetors. Ah, but here's the thing, I would and have and will and do run IDFs. They're simple - primitive in fact - a variation of other single barrel Webers. They pop and snarl and require a good bit of effort, and I like all that stuff. All that said, they're a terrible choice for someone who wants a reliable, start-and-go kind of classic and doesn't want to maintain carburetors.

Re: Weber 34 ADF Tuning

Posted: Tue Nov 28, 2017 6:30 am
by friedman
Give me a box of brass jets over a wiring diagram and volt/ohm meter anytime.....except for my daily driver.

Re: Weber 34 ADF Tuning

Posted: Fri Dec 01, 2017 8:59 pm
by davedecker4
Amen!
I'm even okay with points I can repair with items left in the bottom of the glovebox like a nail file, book of matches and a butter knife. Simplicity and then rewardingly, it runs!

However, unlike Carl I prefer the heat that always pours onto my feet instead of never does. But to each their own