Weber 34 ADF Tuning

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covesurfer
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Weber 34 ADF Tuning

Post by covesurfer » Fri Nov 24, 2017 12:56 pm

Hi All,

'77 spider with stock engine. I have just replaced my 32 ADFA with a 34 ADF which I cleaned and rebuilt. I am having trouble getting it to idle below 1,000 RPM, even with the idle speed screw backed all the way out. When I turn in the mixture screw, it runs smooth until just about seated, then stumbles. If I screw out the mixture screw just a bit from seated, idle will slow down, but seems like that is a hack. I've read elsewhere that I should size my main idle jet smaller so that I can get smooth running when the mixture screw is backed out around 1.5-2 turns, so am planning to change the main idle jet from 50 to 47. I realize that should help me with mixture, but will it help with getting the idle speed right? If not, what are other things to look at? Note, I don't think I have a vacuum leak to the brake booster as I didn't have this issue with the ADFA, but will take a look at that. Are there any other gotchas?

Note, I am planning to go over the jetting based on these numbers, which were provided in another post here. In most cases I am within one step of most of these, but will double check. Also does that number for secondary idle jet look right? I checked Pierce Manifolds and other sites, but can't find any source selling one that large (90):

Main venturis 24/26
Aux Venturi 4.5/4.5
Main jets 120/140
Emulsion tubes F11/F75
Air Correctors 140/180
Idle jet 47/90
Pump Jet 40
Float Valve 1.75

Once I get idle speed right, my next challenges are:

- Flat spot when I accelerate from stop. Feels like it is going to stall when I press the throttle.
- Hesitation / jerking at around 4,000 RPM when under load. I had this issue with the ADFA when fuel filter was gummed up, and was able to address by replacing the filter (I am planning an overdue cleanout of my fuel tank to address at the source). I now see this pretty consistently with the 34 ADF. Not sure if/how it is related. I have a BCD fuel pump that I am planning to change out with a Facet cube style pump.

Any advice would be appreciated!

Thanks!
Daniel
Daniel
Frisco, TX
1977 124 Spider
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friedman
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Re: Weber 34 ADF Tuning

Post by friedman » Fri Nov 24, 2017 3:09 pm

Idle jet size will not affect idle, that's why there is an idle mixture screw to adjust the gas in the idle circuit. Idle jet is sized to make transition from idle to main circuit work properly. Turning in idle mixture screw in all the way should kill the motor. Likewise, turning the idle speed screw all the way out so it is not touching the tang should kill the motor.

Unhook the linkage from the carb to make sure the linkage isn't pulling the idle too high. Make sure the choke is not on which means the motor has to be warmed up with the choke totally off.

If while the motor is idling you see gas dripping out the aux venturi nozzle onto the throttle plate, your motor is trying to idle on the main circuit, not the idle circuit.

If still idling too high or you cannot kill the idle with the speed or mixture screw then you have an air vacuum leak. You can spray brake cleaner around any gasket surface and if the idle goes up you have found your leak. Likewise you can use WD-40 or even unlit propane.
Carl in Virginia
80 spider
86 Bertone
77 X1/9
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bartigue
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Re: Weber 34 ADF Tuning

Post by bartigue » Fri Nov 24, 2017 10:08 pm

I hate to disagree with my friend Carl but the primary idle jet size does have an impact on idle speed, and a 47 is small for an 1800...I used 47s on 1438s. Try a 50 or 52.

Fix the idle issue first then diagnose the rest.
-----------------------------------------------------------
Brad Artigue
1982 Spider 2000
I've restored, worked on, sold, bartered, and bled for:
1970, 1979, 1980, 1981, 1982 124 Spiders -- 1969, 1970 850 Spiders -- 77 X1/9
http://fiat.artigue.com
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vandor
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Re: Weber 34 ADF Tuning

Post by vandor » Fri Nov 24, 2017 10:29 pm

Be absolutely sure you do not have any vacuum leaks.
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Re: Weber 34 ADF Tuning

Post by bartigue » Sat Nov 25, 2017 7:23 am

Note that a vacuum leak typically results in a rising and falling of rpms
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Brad Artigue
1982 Spider 2000
I've restored, worked on, sold, bartered, and bled for:
1970, 1979, 1980, 1981, 1982 124 Spiders -- 1969, 1970 850 Spiders -- 77 X1/9
http://fiat.artigue.com
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friedman
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Re: Weber 34 ADF Tuning

Post by friedman » Sat Nov 25, 2017 9:03 am

I usually don't take issue with anyone disagreeing with me on a technical point but while I wait for the garage heater to warm up my garage I did some homework on whether an idle jet size directly impacts proper idling.
My point was you fit the idle jet to make transition properly function and then use the mixture screw to set idle mixture, you do not change idle jets to make a car idle better.

To paraphrase John Passini's Weber Carburettors:
As the idling jet deals with progression as well, it is chosen for the later function (transition) and the adjustable mixture screw meters the idling.
In practice, any idling jet that gives correct progression will be able to look after the idling with the mixture screw.

Pat Braden in Weber Carbureors:
Thus it is important to change the idle jet and not adjust the idle mixture screw to trouble shoot the progression circuit.

OK, I'm done on this, the garage is warm enough to go install the heater in my X1/9 café racer.
Carl in Virginia
80 spider
86 Bertone
77 X1/9
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covesurfer
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Re: Weber 34 ADF Tuning

Post by covesurfer » Sat Nov 25, 2017 1:42 pm

Thanks Carl and I certainly didn't mean to start any controversy and thanks for the detailed explanation :)

I poked around this morning and it does look like I have some vacuum leak issues based on spraying carb cleaner around the sensor that is between the carb and engine on the manifold. I also blocked off the vacuum port that connected to the air pump mechanism mounted over near the coil. Moral of the story is I am going to go ahead and get rid of all the smog stuff and start my troubleshooting from there.

Here are things I was able to confirm:

- Linkage was not binding (I have the cable type)
- Fuel was not leaking from the venturis (they were spitting as soon as I opened the throttle)
- Engine runs fine but slows (doesn't die) with mixture screw all the way in. Same with idle speed screw all the way out.

Question: There was a definite sputter/near stall from when I opened the throttle idling at around 1,100 RPM and before the secondary opened. I assume that even once I get the vacuum issues sorted, this will still be an issue. I have a 50 idle jet, and need to confirm what my primary main jet is, but when you talk about progression, would this symptom (flat spot when I apply throttle) mean that I need to go larger on the primary main jet? Or?

Learning a lot here, thanks for your patience!
Daniel
Frisco, TX
1977 124 Spider
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Re: Weber 34 ADF Tuning

Post by friedman » Sat Nov 25, 2017 3:36 pm

You need to solve the vacuum leaks first, then see how the motor performs. Also, pull out the idle jet and make sure it's not plugged with dirt.
Carl in Virginia
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86 Bertone
77 X1/9
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Re: Weber 34 ADF Tuning

Post by bartigue » Sat Nov 25, 2017 4:25 pm

Carl, the only person you disagreed with by quoting that part of Passinis book was yourself; either that or I can’t understand your initial post (which I’ll admit). The limit of mixture adjustment is determined by the idle jet size (the primary jet).
-----------------------------------------------------------
Brad Artigue
1982 Spider 2000
I've restored, worked on, sold, bartered, and bled for:
1970, 1979, 1980, 1981, 1982 124 Spiders -- 1969, 1970 850 Spiders -- 77 X1/9
http://fiat.artigue.com
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Re: Weber 34 ADF Tuning

Post by friedman » Sun Nov 26, 2017 9:13 am

We can discus it over tacos and beer the next time you are in town. I like giving you a hard time whenever I can, a sure sign you are a good friend of mine.
Carl in Virginia
80 spider
86 Bertone
77 X1/9
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