William2 wrote: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.
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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.
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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.
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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.
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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.
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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