Dellorto Idle/Progression/Mains

[William2]

I am interested to know how the idle, progression and main jet circuits work together on Dellorto carbs. Is it the case that the idle jet + idle jet holder in conjunction with the progression circuit controls the fuel/air mixture up to about 3000rpm before the main jet air air corrector have any affect at all? If this is true, what controls when the main jet kicks in?

[Craven]

William,
All the information you seek is available in the definitive publications from the manufactures, simple interpretations of the transfer stage of these carburettor by various contributors only deal with the very basics.
Little, if ever, is discussed about the function of the emulsion tube and it?s importance during the transfer stage. If there is someone on this forum who is prepared to give a detailed explanation then it may help you, but from your posts it would appear taking reference from a recognised publication is your way forward.
Ron.

[john.p.clegg]

I think it would be safer to talk about throttle opening positions rather than RPMs..

John
Just refitted the Dellortos after 10 years or so of EFI....

[alan.barker]

So what was wrong with the EFI
Alan

[john.p.clegg]

Nothing , from 29 mpg to 39 mpg with no losses ( of fun ) it's going on my Morris Minor...but the throttle bodies airbox etc are up for grabs.

John

[Esprit2]

I am interested to know how the idle, progression and main jet circuits work together on Dellorto carbs. (Snip)...

The Idle Circuit's Mixture Screws and the Idle Progression Ports break into the throats down stream from the throttle butterflies (ie, on the engine side of the butterflies). With the engine running and the butterflies closed, those ports are subjected to strong manifold vacuum (engine sucking on a closed manifold runner) that basically 'sucks' the fuel and air through the Idle Jets/ Idle Air Correctors/ Emulsion Tubes, and into the throats. The motive part is as simple as sucking on a straw.

The Main Circuit's venturis are located upstream from the throttle butterflies, on the air filter/ atmosphere side of the butterflies. With the throttle closed at idle there's very little airflow through the throats and venturis, certainly not enough for the venturis to generate a meaningful amount of vacuum. The Main circuit is effectively 'off' when the butterflies are closed.

As the butterflies progressively open, the vacuum-generating effect of the engine sucking against a closed throttle gets progressively weaker. Try sucking a milkshake through a straw. Now use a needle to poke a hole in the side of the straw and try again. Now poke more holes and keep trying. The more holes you poke, the more outside air leaks into the runner, and the harder/ less productive your efforts are at drawing the milkshake up the straw. Same in the carbs... the more the butterfly opens, the less productive the Idle Circuit becomes.

At the same time, the more the butterflies open, the more airflow there is going through the venturis, and the stronger the vacuum that they produce.

So, as the butterflies open, the Idle Circuit vacuum diminishes, while the Main Circuit vacuum increases. If you graph the two effects, the curves will cross, and the majority fuel mixture flow will follow whichever curve is the highest at any given time... Idle initially, then transitioning to Main. The graph cross-over point corresponds to the transition rpm for those carbs on that engine.

All the above is a gross over-simplification, but it paints a fundamental picture of what's going on.

*~*~*~*
The Jet, Emulsion Tube and Air Corrector fit together to form a 'stick' assembly, and that stick screws down into a jet well in the carb.

The top of the well is open to air at the top, where the Air Corrector lives.

The bottom of the well is open to a passage that connects to holes through the bottom of the float bowl. Natural gravity flow from the bowl fills the jet wells with fuel to the same level as in the float bowl. Everything is flooded to the same level. The jet is in the bottom of the 'stick' assembly, down at the bottom of the flooded well, where it's always immersed in fuel.

A passage is drilled such that it intersects the jet well part way up it's side, at some pre-determined height above the fuel level. The passage angles down to the carb thoat(s) where it's subject to vacuum... ie, the passage is the 'soda straw' the throats are sucking on. Vacuum from the throat is applied to that drilled passage, and tries to suck whatever is in the well (both air and fuel) up, out and into the throat.

Just a little redundancy to pull it all together... The flow path is fuel into the emulsion tube through the Jet in the bottom, and air into the Emulsion Tube through the Air Corrector at the top. The air and fuel flows meet in the middle of the emulsion tube, and burst out through the small holes drilled around the middle of the emulsion tube. As the air and fuel mix pass outward through the holes, they mix and sputter out as a froth... ie, the 'emulsion'. A splatter of small bubbles rather than just a solid stream of fuel. The fuel atomization process starts there.

So, the froth has now sputtered into the gap between the inside of the well, and the outside of the emulsion tube. And it's above the flooded well's fuel level, but below the spill-over height into the drilled passage to the throat. So the applied vacuum must only be strong enough to lift the 'lighter' fuel froth, not the 'heavier' wet fuel, up to the spill-over into the drilled passage, and down into the throat. It's the emulsion that is delivered to the throats.

Shake your Coca-Cola (or your beer). The foam 'head' that develops has about the same consistency as the emulsion. The light weight emulsion is much more easily sucked up to the spill-over point, into the drilling, and down into the throat.

*~*~*~*
The Float Height controls the level of the fuel in the float bowl and jet wells. The vertical position of how high above the fuel level the drilled passage intersects/ breaks into the jet well determines how far the vacuum must lift the froth before it can spill-over into the drilled passage to the throats. It's that vertical dimension above the fuel level, ie, the 'lift height', that determines how strong of a vacuum is required to get fuel emulsion flowing. That determines when the weakening Idle Circuit stops flowing, and when the strengthening Main Circuit starts flowing... ie, the Transition rpm. The designer can choose both the fuel level and the 'lift height', and thereby determine the carb's Transition rpm. Dellortos are about 3200 rpm by design, and Webers are about 4000 rpm.

For the Idle Circuit, the spill-over/ drilled passage goes to another horizontal passage/ tunnel. It runs just above the throat, and has a common wall with the throat. The Idle Progression Holes and the Idle Mixture Screw holes are drilled through that common wall into the throat. Emulsion is drawn through those holes into the throats. The Progression Holes are the Idle Circuits main feed points, and the Idle Mixture Screw is just a fine adjustment. The mixture must be 'right' via the jet selection, and just tweaked a little by the screws... as required.

For the Main Circuit, the spill-over/ drilled passage goes to the venturi and into the throat. That could be a simple venturi in some carbs, or a more complex venturi/ auxiliary venturi combination, as in the Dellorto DHLA and Weber DCOE.

*~*~*~*
The designer can control the fuel level via a specified Float Height setting (it's your job to follow his instructions) and the spill-over height (how high up the well the drilled passage intersects it), and thus how much vacuum each well (Idle & Main) requires in order for it to draw-in the air/fuel emulsion and thus be active. With falling vacuum in the Idle Circuit, rising vacuum in the Main Circuit, and control over the fuel level and spill-over heights, the designer can determine the rpm at which one circuit gives up, and the other starts.

*~*~*~*
Fuel level is critical, since it's the vertical distance from the pool's surface to the spill over height that determines how strong of a vacuum is required to draw emulsion into the drilling. Raising the fuel level by setting the float too high causes an overall rich condition. Setting the float/ fuel level too low causes an overall lean condition. It's possible to do a perfect job of jetting the carbs, then screw it all up by setting the float height (ie, fuel level) incorrectly.

Several Float Weights are available for Dellortos (only one weight for Webers). Each float weight has it's own specific Float Height setting. All of the Float Weights, when accurately set to their own specified Float Heights, will produce the same design-correct Fuel Level. Heavier and lighter floats respond differently, and can impact engine response, etc. That's a level of fine-tuning available with Dellortos (to those who know what their doing) that's not available with Webers. But different Float Weights influence other peripheral tuning matters, NOT the Fuel Level. Properly adjusted, all Float Weights produce the same standard fuel level. As parts availability becomes an issue, if you are forced to install a different weight float in your Dellortos, then you must also alter the Float Height setting as required to maintain the same Fuel Level. THAT's IMPORTANT !!.

Having said that, Lotus was not above taking a few liberties, and some of their Dellorto set-ups use higher Float Heights in order to promote an overall richer condition. So if you see inconsistencies between Lotus and Dellorto specs, it may just be that Lotus was screwing around a bit. They did that with Webers, too.

*~*~*~*
If, during a full throttle run, the engine stumbles just prior to the transition rpm, then the Idle Jet is too small and the Idle Circuit is running out of capacity before the Main Circuit takes over. Go larger on the Idle Jet until the stumble 'just' goes away... no larger.

If the engine doesn't stumble during the full throttle run, then go smaller on the Idle Jet until the engine does develop a stumble. Then go back larger again, one jet size step at a time, until the stumble 'just' goes away. The Idle Jet should be 'just' large enough to eliminate any transition stumble, and no larger.

If the Idle Jet is too large, it will just complicate your attempts to similarly select the Main Jet size. One circuit should be running at a time, with minimal overlap. If the Idle Circuit continues to dump fuel in beyond the transition rpm, then the mixture will become too rich, the engine will start blowing black smoke, bog-down, and loose power. If you mis-diagnose why the engine is running rich, and reduce the Main Jet size instead of the Idle Jet, you'll just be screwing up the mixture the engine needs for high rpm power. Get the Idle Circuit 'right' before attempting to jet the Main Circuit !

Clear as mud... ??

Regards,
Tim Engel

[richardcox_lotus]

Great write up Tim.

Regards
Richard

[William2]

Hi Tim, Many thanks for your very comprehensive and easy to comprehend description of how the carbs work. I have Des Hammill's book but it doesn't cover everything. I have found your help to be invaluable so thanks once again. Regards, William.

[Chancer]

Me too, I read every book I could get my hands on over the years and that is by far the easiest to understand explanation, either that or I read more carefully and have more intelligence with age, it aint the latter

Could you further explain something that I understand but not exactly how it works, the holes in the emulsion tube are progressivey exposed as the fuel level in the well falls to admit more air to avoid a richening mixture with increasing RPM, what makes the fuel level in the wells fall? It cant be a restriction or the well would run dry at constant revs, I guess it has to be venturi vacuum related, can you explain that in your superb way?

[Esprit2]

Could you further explain something that I understand but not exactly how it works, the holes in the emulsion tube are progressivey exposed as the fuel level in the well falls to admit more air to avoid a richening mixture with increasing RPM, what makes the fuel level in the wells fall? It cant be a restriction or the well would run dry at constant revs, I guess it has to be venturi vacuum related, can you explain that in your superb way?

Chancer,
I didn't make that crystal clear, did I? Rather than give a supplemental explanation, it's probably best that I edit the original post to (hopefully) be more clear in the first place. I've just done that. Go re-read it and let me know if it makes more sense now. The first part is unchanged, you can just start reading from the...
*~*~*~*
... that separates a couple of paragraphs.

For those of you who thought my original post was too long, I just made it worse. Sorry about the bandwidth.

Regards,
Tim Engel

[Maxxis Bug]

Hello Tim,

Your lesson on Dells was great!!

I have been working on a pair of Dells for a friend of mine and I am suspecting that the passage(s) that supply fuel to the IDLE openings (under the butterfly) may be partially or fully blocked.

Is there a method of checking that specifically?

Matthew

[Esprit2]

?? ... The 'progression holes' ? There are large-ish brass 'screw' plugs near the idle mixture screws... they're the progression hole covers. Remove them and look down into that passage below (mole tunnel). In the floor of the passage, you'll see several small holes drilled through into the throat. Those are the progression holes, and their number varies with different vintage and model of DHLA carb. The early ones had as few as three, while later DHLA 40'E' & 45'E' carbs had up to six.

It should be easy to visually inspect the holes, and clean them if required. Don't do anything that will alter their size... ie, don't clean with a drill bit.

*~*~*
While you're in there, move the throttle linkage and watch the edge of the butterfly sweep past the progression holes. If the shaft is straight (and it should be), then both butterflies on one carb should cross the edges of the same holes in each throat at the same time. If the shaft is twisted, then one throat's butterfly may reach the edge of a hole in its throat before the other butterfly reaches the edge of it's progression hole. If there's a significant difference, then the shaft is twisted, and you'll never get the balance right between the two barrels.

The standard 'book' fix for a twisted shaft is to replace it with a new one.

Having said that, it is possible to un-twist the shaft. I've not done it, but I've heard others say they have.
Use two wrenches, one on each nut on the opposite ends of the shaft. 'Gently' twist a little bit as required to straighten the shaft. Don't do that visually using the progression holes as a guide. Instead, finish rebuilding the carbs, install them, then use a 4-tube manometer to balance the carbs with the engine running... both the two throats on one carb, and carb to carb via the linage coupler's balance screw.

For small imbalance in a single carb, the carbs have air bleed screws. Ideally it will only take some fraction of one turn to balance the two throats on one carb. More than 1.5 turns is too much, and something should be done to fix the root cause problem... which will most likely be a twisted shaft.

I'm not recommending twisting the shaft (the book says, 'don't'), I'm just laying it out there. If you choose to go that way, then you do so at your own risk. The upside is that a twisted shaft is already toast, and should be replaced. You can't really damage it more, and you might get lucky and fix it.

Regards,
Tim Engel

[Admiral]

@Esprit2

I have read your explanation with hugh interest.

And I have a questio, of course.

Car is a Lotus esprit with ? history.

After many attempts I have now got the engine running quite well. Te big tick was to rise the floet level.
Internally it is 27 mm, the floater now stands at 13 mm.

The engine pull off from idle with power, I can cruise with 1500 rpm with no porblem.

BUT: at the moment of takeover from idle circuit to main it is getting rich. brabbel, brabbel

My question: what is the effect of closing the last progression hole?
Will it weaken the idle circuit at it´s end of operation?

Cheers from Germany!
Michael

[Esprit2]

Michael,
The idle circuit feeds the engine up to 3200 rpm, then the main circuit takes over. 3200 rpm is the built-in "Transition Point", and isn't normally adjustable.

If the idle jet is too large, the idle circuit will continue feeding the engine beyond the transition point. But the main circuit will still begin fueling at 3200 rpm, as it should. So the engine ends up being fed by both circuits, gets far too much fuel, and bogs down. If the engine can struggle through that over-rich stage to a higher rpm at which the Idle Circuit finally does stop, then the amount of fuel delivered will drop off to normal, and the engine starts running well again.

From your brief description, the problem with the engine bogging down sounds like a classic case of the Idle Jets being too large, and going smaller with the Idle Jets would be the correct 'fix'.

I strongly recommend that you NOT modify the Progression Holes. The DHLA is very well designed, and randomly modifying the bodies by changing the Progression Holes could cause more problems than it fixes.

The carbs generate vacuum in two ways. Off idle, the engine is sucking against mostly closed throttle butterflies, and in that way generates the vacuum required to draw the air-fuel into the throat via the Progression Holes. As the butterflies open more, that "sucking" method generates less and less vacuum.

As the air velocity through the carbs increases, the venturi starts generating more and more vacuum. If you graph the vacuum produced by the two methods verses rpm, the two curves would intersect at some point. The size, number and placement of the progression holes controls smooth fueling through small throttle openings, but also controls the point at which "sucking vacuum" gives way and allows venturi vacuum to take over. Modifying the progression holes without knowing precisely what you're doing will most likely cause more problems than it fixes. Don't go there.

Which model Esprit do you have? Naturally aspirated, or turbocharged? And which DHLA carbs are installed? Through the years, Lotus used five different DHLA set-ups for the naturally aspirated models, and four set-ups for the Turbos. How are your carbs currently set up/ jetted? Which throat and choke sizes, and which Idle Jet and Idle Jet Holder (ie, Idle Air Corrector) are installed?

What is the elevation above Sea Level where you live? At higher altitudes the air gets thinner, so the carb's mixture gets richer. The standard jetting is correct for Sea Level, and needs to be re-jetted leaner at higher altitudes. You might get by with the stock jetting up to 2500 ft (762 meters). Higher than that requires re-jetting, and the highr you go, the more significant the jetting changes must be. So, what is the elevation where you are, and have the carbs been re-jetted accordingly? The best way to do that is on a dynomometer (rolling road). If you wish to do it the old-school way, then...

Go back to my first l-o-n-g post in this thread, and re-read the last three paragraphs.

The normal procedure for jetting the idle circuit is to set the Idle Air Corrector (aka, Idle Jet Holder) first. When the Idle Air Corrector is properly set, then...

Go for a drive on a secluded, preferrably flat road where you can put the throttle pedal down. The car would rip through 1st gear too quickly for you to judge what was going on with the mixture. So shift into a higher gear at as low of an rpm that the engine will tolerate, then go full throttle.

If the engine stumbles approaching/ prior to 3200 rpm, then the Idle Jet is too small, the Idle Circuit runs out of capacity before the Main Circuit takes over, and the engine stumbles. Go a step or two larger until the stumble 'just' goes away, but NO LARGER.

If the engine doesn't stumble, continues past 3200 rpm, but then goes rich and bogs down, then the Idle Jet is most likely too large, and the Idle Circuit is continuing to fuel the engine after the Main Circuit starts fueling it. And that's what I think is going on with your engine... it's getting too much fuel for a short time after 3200 rpm. Unfortunately, the full-throttle symptoms give no clear indication of how large is too large, so you just have to go to smaller Idle Jets by trial and error. So install a step or two smaller Idle Jets and repeat the full-throttle test until the engine develops a stumble prior to 3200 rpm. You have now gone too small, but at least you know where you are. Now go back larger/ richer again until the stumble just goes away... no richer. If the mixture is a bit too lean above 3200 rpm, then fix that by adjusting the Main Circuit, and leave the Idle Circuit alone.

Given that the topic is Dellortos, maybe we can finish it here... the concepts also apply to Dellortos on Twin Cams. But this is an 'Elan' forum, and we're now discussing your Esprit. If this discussion continues, then at some point we should take it offline out of consideration for everyone else.

The fuel level in the float bowl is foundational. It must be correct before you start changing jets. All the above presumes you have adjusted the fuel level correctly. If it's wrong, then higher is richer, and lower is leaner. And higher is a smaller dimension, like 13mm. Lower is a larger dimension, like 17mm.

Good luck,
Tim Engel

[Donels]

What a brilliant write up Tim. I have read a few books on Weber’s and Dellorto's but none had such good explanations as yours. It now all makes sense. Thank-you.

Dave