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Note on Weber carbs by Mr. Passini

General engine tech -- Drag Racing to Circle Track

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Caprimaniac
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Note on Weber carbs by Mr. Passini

Post by Caprimaniac » Mon Sep 30, 2019 3:37 am

On balancing multi carb setups, and setting idle. Passine say (page 57 of 1993 edition of»weber carburettors»:
‘When the two carburettors appear to be belanced, with the mixture appeating to be about right, then each each air bleed screw should be opened half a turn,...’

Well- I’ve never done this, and this procedure- as I recall- is not included in any other of my Weber books.

What will the effect of openening the air bleed screws be?

I’ve earlier only used those for balancing if any inconsistency between barrels.

Read some yesterday as I’m converting back to 4x webers on my street car. Hoping to get it better than last time, when I got tired of trying to get it correct- and converted to EFI.... Need to upgrade/ fix the linkage as well.
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Re: Note on Weber carbs by Mr. Passini

Post by MadBill » Mon Sep 30, 2019 4:36 pm

My thought is that it is desirable to have some flow through the bleed passages to keep them clear of fuel residue, etc. for potential future need. It's also possible that this results in a effectively longer transition slot/hole sequence being created for off-idle conditions, similar to drilling holes in Holley butterflies.

BTW, as many have discovered, the difference in idle smoothness, light throttle pick-up and even induction noise between "well balanced" and absolute perfection in multi IR carb synchronization is profound.
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Re: Note on Weber carbs by Mr. Passini

Post by lc-gtr-1969 » Mon Sep 30, 2019 5:42 pm

Personally, I would not use the air bleeds for any other reason than balancing the two throats of a weber. By opening air bleeds a little on every throat, you will find high speed synchronisation difficult, although it can help with idle synchronisation. The author may have his reasons for his comment but I just cant see why he says it. He may have used this to compensate for tune condition, ie running slightly richer idles to help with transition, but then offsetting this a touch with the air-bleeds. Webers can have known lean conditions on transition to main circuit and can be a bear to tune around depending on manifold vacuum levels, type of carbs (and the progression hole design for that specific weber) as well as availability of jet sizes.

I have a few rules about webers (or multi carbs in general)-
1) Spend money on quality linkages, no short cuts here...

2) Use a variable synchrometer, high and low vacuum- sync at idle, then at 3000 or 3500rpm. You would be amazed how often carbs can be synched at idle but be out, quite a bit, at 3500rpm. Linkage gemoetry and leaky butterflies at idle are the main culprits.

3) Float levels are critical, there are a few ways to check weber float levels but either way, spend some time here.

4) run a quality regulator and keep it down to 3psi (for webers).

5) If it is used on the street, start with smaller chokes/venturis that you think you will need and work up.
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Re: Note on Weber carbs by Mr. Passini

Post by modok » Mon Sep 30, 2019 8:08 pm

If the moxture was set for highest idle, Opening the bypass screw would lean the mixture.

I don't know what the idea is.

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Re: Note on Weber carbs by Mr. Passini

Post by MadBill » Tue Oct 01, 2019 12:25 am

I have only a passing knowledge of Weber carbs; somewhat more re carbs in general, but doesn't the air bleed circuit draw from the carb bore just above the butterfly, in which case opening it up a turn or two would have no more effect on mixture strength than turning up the idle screw.
Also if opening the bleed screw did lean out the AFR, would not a slight tweak of the mixture screw remedy the situation?

(Full disclosure: I only ever use the bleed screws to compensate for minor bore-to-bore airflow discrepancies within one carb.)
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Re: Note on Weber carbs by Mr. Passini

Post by gruntguru » Tue Oct 01, 2019 12:31 am

Could it be to ensure none of the bypass passages is closed? If a future balance check shows one cylinder flowing more than the others, you will be sure of being able to simply close that bypass down a bit, rather than have to open all the others, then adjust the idle speed to get it back down to where it belongs.

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Re: Note on Weber carbs by Mr. Passini

Post by Caprimaniac » Tue Oct 01, 2019 1:48 am

Great input. If any other reson to open them up, it must be well hidden.

As I undertamd, AFR would go leaner by opening the bypass as this air would not draw a corresponding amount of fuel.

As you state, Bill, Passini too say a small tweak of the idle mixture screw after opening the bypass may be necesarry.

Passini’s writing is good, but he dorsn’t always give the reasoning behind what he states. Nor does he give a recepie on How to set idle, or anthing like that. But it’s all somehow there, hidden in the text...
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Re: Note on Weber carbs by Mr. Passini

Post by enigma57 » Tue Oct 01, 2019 2:19 am

As has been stated by others here, I only use the air bypass screws with Webers on IR intake and then, only to balance them if there is a slight difference in flow between the 2 throats (differences in throttle blade seating due to wear or as a result of a twisted throttle shaft, for instance).

I will add that whilst reconfiguring DCNF carbs for use on plenum type intake (the DCNF was designed for IR intake)...... I did try opening the air bypass screws in an attempt to correct the throttle blades partially uncovering one of the progression holes at idle (tickover), as I could not get idle speed below 1,100 RPMs without uncovering the 1st progression hole.

In so doing, I was surprised to find that opening both air bypass screws fully would not do it. So I closed them both and drilled a hole in each throttle blade. Progressively increased hole size from 1/16" (1.58mm) to 5/64" (1.98mm) to 3/32" (2.38mm) before I managed to get idle speed manageable at 900 RPMs.

This was not a radical engine. This was a road car in what I would call a mild state of tune. Used for weekend trips and occasional rallying. Single Weber 40 DCNF on 1700cc Saab V-4 engine using a modified Saab open plenum intake, headers (extractors) with free flowing full exhaust system and cam of around 220 degrees duration @ 0.050" lift. Later changed to 36 DCNF (car runs best with 29mm chokes and 36mm throttle bores on plenum intake, better than when running 29mm chokes in 40 DCNF). Previous to running 40 DCNF, 36 DCNF and 36 DCNVH...... Ran 42 DCNF with 32mm and 30mm chokes but the 42 was just too large and choking it down with smaller choke tubes...... Did not run as well as the 36 DCNF.

Now running a rejetted 36 DCNVH with 29mm chokes (main venturii) original to an '84 - '85 Euro-spec 2 litre Maserati Bi-Turbo (this carb was designed for open plenum intake and unlike the DCNF carbs, has a power valve). Bi-Turbo PV spring was too long / stiff though, so had to use shorter power valve spring from a crusty old naturally aspirated DCN carb I had here). A bonus...... Because the DCNVH was designed for plenum intake, the relationship of throttle blades to progression holes at idle speed was correct for the application and unlike the 40 DCNF and 36 DCNF, I didn't have to drill holes in throttle blades with this carb. Similar carb as used on 1.6 litre Sunbeam and Talbot, etc., only calibrated differently. Might mention as well...... The DCNVH being designed for plenum type intake, has no air bypass screws.

Best regards,

Harry

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Re: Note on Weber carbs by Mr. Passini

Post by Geoff2 » Tue Oct 01, 2019 6:26 am

The bypass holes are only there for low speed [ idle ] synchronisation. If needed. They have no effect on high speed because the tiny amount of air they add is minute compared to the air at WOT. My experience has only been on V8 engines & I have found that as long as the linkages have no slack, using the bypass screws to add idle air for balancing, makes no noticeable difference to idle speed, or idle quality. Where I have found the bypass holes useful is on some engines with IR you can get trailing throttle richness, resulting in popping/backfire in the exhaust system. Open the bypass holes on each throttle bore 1/2 turn helps/cures the problem.

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Re: Note on Weber carbs by Mr. Passini

Post by JohnP » Tue Oct 01, 2019 11:50 am

I bought his little green book in the late 70's, while working on my '76 Fiat 131. I remember he said your plugs should be the color of Cadbury's milk chocolate. Another gem, "Suck it and see" (to find out if something will work).
Thinking is hard work.

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Re: Note on Weber carbs by Mr. Passini

Post by chimpvalet » Fri Oct 04, 2019 12:06 am

Had both of Passini's books back in the '80's, have to say the writing was more artsy than technical throwing lots of words and phrases around without nailing down the "why" of most matters. Great example was his memorable, "suck it and see" mentioned above. Really solid, methodical work is being done currently by Keith Franck in California. He hosts 2 forums on Yahoo, the Weber focused one being tagged, "Sidedraft Central", where you may find plenty of insight on correcting the deficiencies inherent in the DCOE's above all. Good example is his advice on how to replace tired leather seals on throttle shafts before embarking on futile efforts to tune idle/off idle/balance. Recommended.

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Re: Note on Weber carbs by Mr. Passini

Post by Geoff2 » Fri Oct 04, 2019 6:19 am

Don't know which Passini book you have read Chimpvalet, but the one I have is more than technical, with veeeery well explained explanations, along with some very 'British' comments. An example: what is the purpose of extended auxiliary venturiis, versus std length aux venturiis. The reason is explained, along with the downside it causes.

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Re: Note on Weber carbs by Mr. Passini

Post by jarmoyp » Fri Oct 04, 2019 9:16 am

Trottle is the only air bleed. It is fuel bleed you talking about. No reason to open it more than necessary, if the car goes smooth and well at low rpm and the A/F ratio is correct, don´t screw it more open than necessary.

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Re: Note on Weber carbs by Mr. Passini

Post by lc-gtr-1969 » Fri Oct 04, 2019 9:05 pm

jarmoyp wrote:
Fri Oct 04, 2019 9:16 am
Trottle is the only air bleed. It is fuel bleed you talking about. No reason to open it more than necessary, if the car goes smooth and well at low rpm and the A/F ratio is correct, don´t screw it more open than necessary.
No, some weber carbs (not all), have air bleeds/ air correctors that are used to correct the balance of airflow at idle between the two throats of a weber carb- they are basically used to help offset small differences in airflow when an issue exists within a particular throat of a weber, such as a leaky butterfly or a slightly twisted shaft. These are separate to the fuel screws. The air bleeds introduce a small, controlled but intentional air-leak into one throat of the twin throat carb.

One of the issues with these screws though, as I have seen a few times, is when running multiple carbs, if one throat is reading lower vacuum, it is quite often a leaky butterfly in the sister throat, so whilst its easy to balance the 2 throats by slightly introducing a small amount of controlled leak in the partner throat, a lot of people then fail to test synchronisation at higher rpm, like 3500rpm, where often you now have the affected carb reading different to the unaffected carbs (even though they all sync up at idle).

This is not directly caused by the air-bleed as it mainly affects the idle air draw, however, as one throat of one carb is reading artificially high (due to a fault), tuners can often raise the other carbs (on multi carb setups) to the same vacuum level, however at 3500-4000rpm, the throttle plate leak (or equivalent) is now not an issue and you in effect have one carb now drawing a different amount of air (ie not synchronised) at the higher rpm.

I only mention this because I have seen many multi carb setups synchronised beautifully at idle with the assistance of the air correctors, but then see them quite out at higher rpm. It's just worth noting. Nothing wrong with using air correctors (I use them all the time), but just always keep in mind that there is more to idle synchronisation and when using air correctors, just remember that we are using them to correct a 'fault', and that correction needs to be considered as to whether this will impact the higher rpm synchronisation. The easiest way to get this right is if using quad webers (8 throats) and one throat is reading high, open the air correctors on all other 7 throats so every throat now has a slight 'leak', then proceed to synchronise at idle and at 3500rpm.
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Re: Note on Weber carbs by Mr. Passini

Post by Caprimaniac » Sat Oct 05, 2019 5:36 am

JohnP wrote:
Tue Oct 01, 2019 11:50 am
I bought his little green book in the late 70's, while working on my '76 Fiat 131. I remember he said your plugs should be the color of Cadbury's milk chocolate. Another gem, "Suck it and see" (to find out if something will work).
Anyone who ever had a Cadbury bar, know he's correct on that statement. "Suck it and see"- sometimes appropiate, isn't it?

I'll leave the air bleeds shut.

But IF a small difference in vacuum at idle between barrels will give a lousy driving engine at lower RPM (which you see alot of in traffic) I will use them for sync. Importance of this I do not remember…. Been some years since I used multicarb setups. What I DO remember is that syncing the different CARBS is utterly important...

If you have unsynced barrels at 3000+ rpm, the shafts or bores or blades need to be faulty. OR, the different cylinders might be different regarding how much it sucks….. based on cam lobe, valves, ignition…… alot of different factors that can vary. Either the carb or engine need to be fixed, at a larger scale...
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