Weber IDF progression phase

Road-race engines and ancillaries - general discussion
Urbancamo
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Weber IDF progression phase

Post by Urbancamo » September 11th, 2011, 7:03 pm

Title is a paradox or is it?

I have read similar comment's on various IDF owners ower the web. I don't remember how many progression holes these IDF's had, if im right there is 3 of them. Is there a slightest chance for reasonable progression with 3 holes?

Newer DCOE Webers had 4 progression holes and in special order you might get them even with 5 progression holes! Some Solex carbs had 7 progression holes, definetly no issues with light throttle.
Early Weber IDA carbs had only one progression hole, and there was really no progression with that setup.
IMG_5474 (Custom).JPG
IMG_5474 (Custom).JPG (81.5 KiB) Viewed 10883 times
There is absolutely no other issue with these italian-made IDF carbs than lousy progression phase. It's certainly annoying when you have to add gas lets say 1-2 mm - 2 cms. A/F ratio jumps to 16-17:1 and it stays there a long time.

Specs:

32 chokes
F11 emulsions
50 idles
55 pump return valves

Tested these idle jets:

45 and 47: super-lousy progression and too lean for highway speeds, sputters and sneezes all the time, undrivable

50: reasonable A/F readings under 100 km/h, still little lean, still lousy progression, drivable

55: far better progression, very little hesitation.. but pig rich under 100 km/h light throttle

Haven't tested yet 52 idles that are available...must order them.

Often folks have conversations around pump return valves and their percentage readings, but do they affect so much because problem isn't so much accelerator pump-related?
Currently with 55 pump returns 55% gas will be returned to float chamber. There is 0, 33, 55, 80 available.

When you slam on throttle, it goes like mad, as it should. But light/semi-light acceleration is a problem.

Comments, knowledge from IDF owners and others are welcome.

Tommi
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 15th, 2011, 8:08 am

Pity this attracted no replies - it's a very good report and something seldom tried.

You're right about the uncomfortable progression phase on IDF. I usually just advice drivers to get through it as fast as possible, but then my work is more biased to competition engines and it's way outside the powerband so no reason to 'hover' there.

G

Urbancamo
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Re: Weber IDF progression phase

Post by Urbancamo » September 15th, 2011, 9:34 am

I agree Guy, when driving hard, it doesn't bother at all. And it's acceptable when driving "normally", you just have to add gas rapidly and not to "feather" it...
Only issue is that mixture leans so much and so long time that it will cause light detonation very easily. There is absolute no detonation otherwise, i have used detonation "ear phones" and no detonation on higher rpms. It so audible noise with headphones, you won't miss it.

So i have already made engine bay ventilation better and now making the airbox to duct some cold air for the carbs.

I have a habit to try make things as good as possible, so i made this topic.

In the past i also tested F16 emulsion tubes with IDF's and there is no use for them in Fiat engine!
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Re: Weber IDF progression phase

Post by Guy Croft » September 15th, 2011, 9:52 am

Yes, I'd tend to agree though in fact F16s are not a tube I have ever used in IDF.

The Fiat 1608 124 BC Coupe and 1800 124CSA which had 40 and 44 IDF respectively both came with F11 as standard which is OK up on mildly tuned setups but on the more powerful versions and certainly on any tuned version 2 liter I recommend the richer F9.

Someone told me recently they used F16 with success on IDF but I cannot personally comment.

In case anyone is unfamiliar with emulsion tubes on these downdraft carbs (for inline engine rear-wheel drive applications) the photo below indicates the emulsion tube assembly which unscrews from the carb body. There is one per barrel and at the top under the threaded retaining cap is the air corrector and at the other end immersed in the gasoline in the float chamber is the main jet. The job of the tube is to 'emulsify' the gasoline and air prior to its being sucked out of the emulsion tube enclosure into the venturi.

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S&B125
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Re: Weber IDF progression phase

Post by S&B125 » September 20th, 2011, 7:08 am

I have used F16 with some success in IDF44's on a 1756. I agree that they have no application on these engines with DCOE. The F16 causes a RICH area in the jetting just as the transition phase comes onto the main circuit (2700-3500 rpm depending on throttle position) which upsets things when in DCOE but can sometimes be used to great affect in IDF's. It is basically because the transition mixture is quite tuneable on the DCOE and "near enough will have to do" on the IDF. Try F9 which are quite easy to get hold of because all the early Alfas used them - they work really well in DCOE's.
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 20th, 2011, 8:50 am

"I agree that they have no application on these engines with DCOE"

uh-oh! I always use F16 with DCOE except on low-tune small units!

G

Urbancamo
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Re: Weber IDF progression phase

Post by Urbancamo » September 20th, 2011, 11:21 am

My other friend uses F11 emulsion tubes in his 1585TC Lada equipped with 40 DCOE's / 33 chokes. I have driven with this car a lot, and it has certainly better progression phase than IDF's ever could offer.
These DCOE's are equipped with 3 progression holes.

When i tested F16's in this title car with IDF's, progression phase seemed to disappear. Engine had huge flat spot when accelerating and it canno't be adjusted.
F11 emulsion seemed to solve all other problems, but lousy progression stayed there.

F9 tubes are almost identical than F11, but F9 has 0.2 mm bigger inner bore and F9's don't have the lowest 4x1 mm drillings. Otherwise identical than F11.
I suppose F9 will bring mains sooner in the game than F11.
Last edited by Urbancamo on September 21st, 2011, 10:10 am, edited 1 time in total.
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 20th, 2011, 12:14 pm

Yes, agreed, I would likely use F11 on DCOE 40s on that 1585 too.

G

S&B125
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Re: Weber IDF progression phase

Post by S&B125 » September 21st, 2011, 8:20 am

I see Guy that you have pulled out the "I agree that they have no application on these engines with DCOE" where I was specificly talking about small engines & low state of tune as in this case.
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 21st, 2011, 9:11 am

"I have used F16 with some success in IDF44's on a 1756. I agree that they have no application on these engines with DCOE"

Sorry - I think I have misunderstood your point. One can read it two ways. On re-reading I think you are saying you would not use F16 in a DCOE setup on a mildly tuned 1585? We agree on that.

G

Urbancamo
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Re: Weber IDF progression phase

Post by Urbancamo » September 27th, 2011, 4:56 pm

Little comparison with common dual carbs.

Now i have driven:

1 pair of 40 DCOE
1 pair of 40 Dellorto DHLA
1 pair of 40 Dellorto DRLA

and

1 pair of 40 IDF

Speaking honestly, IDF's are the worst of these. Cannot be compared to others.

DHLA pair was great. Even the mixture got little lean and jumped to lambda=1 range when accelerating lightly, it doesn't bog down like IDF's. It just goes.
With IDF's, when mixture goes even near lambda=1, it dies and stutters like mad and this lasts long.

Usually this happens in uphill when load goes higher. This lean mixture and semihard load leads to detonation very easily on relatively low-compression (9.5:1) TC. Even with 98 RON octane and very mild ignition advance.

I must consider some fix for that, real annoying thing.

Tommi
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 27th, 2011, 5:00 pm

Good report Tommi but something definitely not quite right there on the IDF they shouldn't be near so bad as to cause detonation, check it out.

Pump jet malfunction?
- Worn pump jet diaphrams?
- Blocked float chamber bleed valve?

G

Urbancamo
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Re: Weber IDF progression phase

Post by Urbancamo » September 27th, 2011, 5:30 pm

This IDF pair has been like this for years. I've carefully disassembled carbs and serviced them in the past. They are in great condition still. All working.


Firstly they had 28 mm chokes. Too small for top-end power, but little better progression. Then changed to 32 mm chokes. Much more power but flat spot got even worse.

I just remember that in the past i tested 0% bleedbacks (100% fuel to the accelerator pumps) with these. Found no improvement, only overfuelling the engine while accelerating.

I've also tested various accelerator pump adjustments and found very little of none effect.

With 55 idles fuel just covers the progression phase (or transition phase, which one you want call that) reasonably smoothly, but then mixture is pig rich while cruising (10-11 AFR, lambda 0.68-0.74)
With IDF's you are stuck with that jet size. DCOE's have these great F-numbers to play with.

So the real problem is, main jet circuit comes in REALLY slow. That's easily seen in lambda gauge while driving. I don't know what causes it. Do engine have a weak vacuum or something else.
Currently stock cam timing is:

Inlet 2 mm LATDC
Exhaust 1 mm LATDC

If someone has thoughts about cam timing and this issue, you're thoughts are welcome.

Maybe next i'll test 52 idles, if they don't work, i'll have to try size up the 55 idle jet air holes to get leaner mixture. Like DCOE's with different air holes.

It's certainly a challenge to try to suit race-designed carbs in civilian use, even if i'm ready for huge compromises.

T
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Guy Croft
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Re: Weber IDF progression phase

Post by Guy Croft » September 28th, 2011, 8:50 am

I do like your reports Tommi, and given that English is not your 1st language they are quite remarkable really.

Re:

Speaking honestly, IDF's are the worst of these. Cannot be compared to others.

DHLA pair was great. Even the mixture got little lean and jumped to lambda=1 range when accelerating lightly, it doesn't bog down like IDF's. It just goes.
With IDF's, when mixture goes even near lambda=1, it dies and stutters like mad and this lasts long.


All in all I guess you are right then!

My advice on IDF? Don't hover/drive in the progression phase - 'punch' thru it asap!

G

robert kenney
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Re: Weber IDF progression phase

Post by robert kenney » September 28th, 2011, 5:36 pm

Might I ask what your initial timing is set at?
While you want 34-36 total with large cams initial will want to be 18*. Lazy low speed caused by big cams will induce large throttle openings that uncover all of the progression holes before you have enough air flow to pull fuel.

You mention increasing idle air bleed. You won't drill the emulsion holes in the 55 jet itself but the master bleeds in the main carburator body. This is common practice in high performance applications. With 55's i assume you have little idle screw opening. Adding air bleed will allow a larger screw opening and lean the cruise richness you are having with the 55's.
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