Getting rid of auxiliary venturis

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Re: Getting rid of auxiliary venturis

Post by bartigue » Tue Oct 08, 2019 7:48 pm

If your mixture screws are all the way in and the car still idles then you are idling off of fuel coming from somewhere up the progression holes. Which could be the result of several things, including the wrong venturi size (too small) creating too much air velocity at too low of a speed. That's one end of the extreme. The other end is both carbs are over-jetted on the mains and under-jetted on the idles.

All of the above remind me why I don't have IDFs right now.
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Re: Getting rid of auxiliary venturis

Post by fiatfactory » Tue Oct 08, 2019 8:16 pm

IDF's have air bypass ports. During periods of low engine speeds air is drawn into these ports, around the butterflies, to provide a lean mixture. Moving the air bypass screw outwards (counter clockwise) will increase the flow of air around the butterfly. Th air bypass ports do not affect mid range or high speed operation. These ports can also be used to equalise the air flow through the carburettors at slow idle speed.

Axiliary venturis are listed with numbers such as 3.5, 4.0, 4.5 etc, these numbers refer to the smallest cross sectional area in square millimeters of the central duct where the fuel is drawn through. A smaller cross section will provide a larger "pull" on the fuel and increases the emulsion capabilities of the emulsion tube, which will result in an overall leaner mixture. A practical application of this fact would be if your calibration was generally rich, and you could lean the entire range with a change to a smaller sized aux venturi.

Those "kits" have been around for a long time...and yes it's one way to actually get a 34 venturi in a 40IDF to actually work properly (if you haven't noticed the kit increases the size of the main venturi to 34mm) and yes the standard 40IDF auxiliary venturi is VERY restrictive to arflow, as it has a full circular design which significantly reduces the overall cross sectional area available for air flow... PLUS... the inside diameter is only abut 36mm (and again the central bar/cone will again reduce available cross sectional area) so if 34 chokes are fitted to a 40IDF with stock auxliliary venturis then there simply is NOT ENOUGH VENTURI ACTION to speed the air flow thru the main venturi, there are no if's or but's on this, that is what happens. These kits reduce the air flow restriction caused by the aux venturi, but I would be sceptical about their ability to atomise the fuel as well as the stock method, and complete atomisation will give a much leaner AF reading.

A common problem with old IDF's is leaking enrichment plungers (choke operation) which will provide LOTS of additional fuel in addition to what the idle circuit s providing... sounds to me like this is your most likely problem, and easily fixed.

SteveC
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Re: Getting rid of auxiliary venturis

Post by ward00 » Tue Oct 08, 2019 9:43 pm

SteveC - are the plungers that you are referring to the two tubes that spring up and down within the choke assembly. If so, I have the chokes disabled/blocked off with plates. Should the plungers still be an issue, if so, how exactly do I disable them, plug them?
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Re: Getting rid of auxiliary venturis

Post by fiatfactory » Tue Oct 08, 2019 10:11 pm

ward00 wrote:
Tue Oct 08, 2019 9:43 pm
I have the chokes disabled/blocked off with plates. Should the plungers still be an issue, if so, how exactly do I disable them, plug them?
By fitting plates over and disabling the actuating mechanism of the cold start enrichment circuit you don't block off the plungers, and if they were leaking before, then they will still be leaking now.

Only way to block them properly is to remove the carbs, and from the underside tap the discharge port - it's a very small size I use something like M4 x 0.5 thread and then fit a very small grub screw which you Loctite in.

Fuel spilling from enrichment plungers you can't see, it's under the throttle plate.

Fitting a smaller idle jet and going richer on your AF reading only happens if the idle jet is physically too small, and then the fuel gets drawn thru the main circuit...resulting in large drips from the aux venturi and this unatomised fuel tends to not burn well and a high AF is the result, these you can see as evaporating spots on the closed throttle plates.... the fact you can close the idle mixture screw all the way in confirms this is likely happening, and also indicates there is additional fuel coming from somewhere, and with IDF's that's more often than not the enrichment plungers not sealing/seating properly.

The extra fuel at idle CAN NOT come from the progression holes as the throttle blades are not being tipped and exposing the progression circuit to the airflow.

and BTW that kit won't be a direct fit with the 13/15 type webers you have... as they are roller type pump levers... which the fitting instructions say need modification to the kit to work... I have no idea what that modification is and they don't say either.

Fitting 00 pump bleed back to cure the transition stumble is total BS. This issue happens during the brief period of time where BOTH the progression and main circuits are providing fuel, and the period is so brief and constant and it has absolutely nothing to do with the pump circuit, as it happens when there is no additional throttle input... i.e. with a steady throttle when the pump circuit is simply not operational.

The brief period of time where both the progression and main circuits are flowing is probably the most critical in the function of the carburettor. During this time the slightest miscalculation in calibration can result in an exceptionally rich engine condition, causing the engine to hunt or surge. This can be caused by idle jets that are too large, mixture screws set too far out, float level set too high and even too much fuel pressure.... in your case probably due to being excessively rich as the enrichment circuit will be constantly spilling fuel, combined with that brief period where both progression and main circuits are working and tending to go rich anyway.

SteveC
Last edited by fiatfactory on Tue Oct 08, 2019 10:45 pm, edited 1 time in total.
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Re: Getting rid of auxiliary venturis

Post by ward00 » Tue Oct 08, 2019 10:44 pm

So my fellow Fiat Club members that have driven behind me will no longer be able to complain about the fumes - excellent :D . I have had this rich issue for the longest time, perhaps it will now be over. I assume I'll get a little bet MPG as well.

As for roller type carburator, my carbs have rods with nuts on the end of them, these are not the roller type that you mention, are they?
Currently in Santa Cruz, Ca
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Re: Getting rid of auxiliary venturis

Post by fiatfactory » Tue Oct 08, 2019 10:49 pm

if you pump levers have rods with nuts on them, then they are not 40IDF13/15 carbs.... they are a different model of 40IDF than what was used as standard on the 124BC.

00 pump bleed valve effectively will make the pump circuit richer... opposite of what you want to cure the transition stumble

a copy and past from pegasus racing as I couldn't be bothered to write this myself...

The accelerator pump dischage valve (also called the intake and exhaust valve) bleeds off some of the accelerator pump output back into the float bowl. This controls how much fuel is actually delivered to the pump jet. Once you have selected your accelerator pump jet size, you can use this valve to fine-tune the output of the pump. If the accelerator pump jet is a little too large, one of these valves can get you to a setting "in between" jet numbers.

Discharge valve sizing, similar to Weber jet sizing, refers to the bleed hole size in .01mm increments. The 40 discharge valve bleeds fuel back into the float bowl as if it had a 0.40mm hole. The 100 size bleeds like a 1.00mm diameter hole, delivering the least fuel to the jet. (The 100 size makes a 50 size accelerator pump jet behave more like a 46 size.) The 000 size has no bleed hole, so it forces all of the accelerator pump volume to the accelerator pump jet.

The closer your accelerator pump jet is to being too large, the larger the discharge valve required. The closer the jet is to correct, the smaller the valve required.

SteveC
Last edited by fiatfactory on Tue Oct 08, 2019 10:58 pm, edited 1 time in total.
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Re: Getting rid of auxiliary venturis

Post by ward00 » Tue Oct 08, 2019 10:54 pm

Thanks for the new info, I'm not sure I ever saw the pistons being addressed as part of the disabling of the choke in any of the articles I read, they all seen to say remove the choke housing and replace it with a plate. Just when I thought there was no more to try, to do, and to learn :twisted:

As for the 00 bleedbacks, I'm agnostic, but I think some people here are not.

This article seems to address the procedure http://forum.porsche356registry.org/vie ... hp?t=19426
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Re: Getting rid of auxiliary venturis

Post by fiatfactory » Tue Oct 08, 2019 11:04 pm

blocking the enrichment discharge orifice is only needed if the plungers are not seating / sealing.... only happens in old carbies, new carbs will seal off completely.... internet info provided will assume they were actually sealing, I know from experience that they tend to leak.

All it takes is a little of the white powdery corrosion down in the base of the enrichment plunger holes, they only seal on a tapered face (like an engine valve) with a very light spring to hold it closed. There is a special tool to recut the tapered face in the carb body, looks a bit like the tool to cut a new sealing face on your water faucet / tap (but who bothers to do that anymore) but very tiny, I've only ever seen pictures of the EP taper seat cutter, never seen one in real life in my 35 plus years playing with weber carbs, so chances of finding someone with the right tool are pretty slim.

the description in the porsche article locks the plungers down from the top, I guess that could work too, plugging from the underside is reversible... not sure if removing some parts and forcing the taper to seal is the best way to do it... but I guess it can be done from the top without removing the carb which would appeal to some people, if the taper was really bad I would guess it could still leak.

if you search the T124 and other forums I'm positive you will find I have described this procedure several times in the last 15 years...

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Re: Getting rid of auxiliary venturis

Post by friedman » Wed Oct 09, 2019 8:21 am

The bottom line for the moment is to figure out why your car idles with the mixture screws completely closed as this is not proper. It could be something as simple as the linkage is too tight and is pulling the throttle open with your foot off the gas pedal so you are on the main circuit, the way to check for this is to undo the linkage to the carbs....on my IDFs if the throttle is open too much at idle and trying to idle on the main circuit rather than the idle circuit then I see drops of gas coming out the aux. venturi onto the throttle plate and cause a slightly erratic idle.

I have blocked the cold start on all my IDFs by threading the discharge port on the bottom of the carb for the cold start circuit and running in a grub screw. You seem to have the newer style IDFs with the threaded rod actuating the accel pump and I have no experience with them.
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Re: Getting rid of auxiliary venturis

Post by fiatfactory » Sat Oct 12, 2019 6:23 am

biggest problem with going to a larger than 32mm choke size (which also happens to be the largest size weber supplies as an original part for the 40IDF) in a 40IDF is the very restrictive nature of the 40IDF's auxiliary venturi...the ID of the 40IDF aux venturi meausres just 37mm

To assist those who have never had these apart before, I snapped some pictures to show the differences, and how I have gone about modifying them for the last 25 or 30 years...

here's a picture (worth a 1000 words) of a standard 40IDF aux venturi on the left, a 44IDF aux venturi in the centre, and the start of modifying a 40IDF aux venturi for more airflow and allowing me to fit a 34mm main venturi on the right.
20191012_181509.jpg
20191012_181509.jpg (361.95 KiB) Viewed 53 times
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