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Discussion Starter · #1 · (Edited)
Hi all,

I wonder if you can shed some light? I am driving a Spider 2000 Veloce 86 Italian Import.

Problem: Since changing my carbs from Solex to Dellorto's I have had a problem where the car can drop revs if cold at around 2500rpm. It literally slows down. When warm it tends to hesitate or bog down when the throttle is approx quarter open. I changed carbs because the Solex were misfiring a bit and was told Dellortos are generally better and more tuneable.

The problem sort of disappears if you drive the car hard but I still believe I am losing some vital HP and the acceleration is intermittent. The DHLA40H are tuned and it runs smooth at idle, these were off a 2000 GTV.

The carb mounts were changed as we thought it may have been sucking in air and a mechanic said that this can cause a problem. Since changing these the problem isn't as extreme as before but it's still there. The timing has been done with a strobe so should be spot on.

Could a problem like this occur through incorrect jetting? I am slightly sceptical because 1. It came off practically the same engine 2. after 3000rpm it's a lot smoother.

Car is jetted as follows:

Emulsion Tube: 7482.3
Idle Jet: 57
Main Jet: 148

Would be grateful for some tips / insight.

Thanks
 

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.3 tube seems a bit lean... AFAIK there were 8,10,or 11 used with H carbs
 

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oops... rsordillo, it looks like you're speaking about "emulsionatore avviamento" - that is "starter emulsion tube", i mean, the cold start device, not the main one. That's what 7482 indices read. So I'm wrong about the "lean" main stack. And you said it's smooth after 3K, so it's hardly the main jets' issue. It still feeds mainly through the idle jets and progression holes at 2500.
But try starting with simple things first: float setting, needles, needle filters, duct cleaning...
 

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Discussion Starter · #6 ·
I have changed the gaskets, cleaned out the jets etc. I set the floats to 16mm with some callipers.

It's not completely smooth after 3000rpm just better. Carbs are balanced nicely. The odd thing is that when you first accelerate away after a cold start it really drops in revs like at least 500 rpm for a few seconds before it then accelerates away with a bit more gas. It's an odd problem I have never encountered before.
 

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About the 16mm:
I have encountered 14.5-15.0 mm and 16.5-17.0mm, depending on the weight of the float.
Any reason to go for 16mm?
 

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Discussion Starter · #8 ·
Actually it was 15mm I set it to, I got it from a few threads and a bit of research online.

Still i don't think it's the float level that's giving me this problem. It's weird because at 2000 RPM and less than half open throttle it just crackles and bogs down. Like if it hits a wall. I may have to take some videos and upload.
 

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Does that happen when accelerating, or when you keep the revs constant at 2000?
First is an indication of the accelerator pump, the second of a mismatch when going from progression to main.
 

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Discussion Starter · #10 · (Edited)
Does that happen when accelerating, or when you keep the revs constant at 2000?
First is an indication of the accelerator pump, the second of a mismatch when going from progression to main.
When i accelerate from idle to about 2k it just seems to nose dive when cold and hesitate when warm, would this indicate a jetting issue or accelerator pump problem?

I have installed trumpets since the carb change so maybe theres too much air relative to fuel at low rpm?
 

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Richard Jemison
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Carb issue

Check the diaphrams in the carbs. If good then pull the carbs and see if there is actual spray from the acellerator nozzle.
 

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Yes, trumpets will influence jetting.

Can you indicate the exact jetting you are using?
The 7482.3 emulsion tube you indicate is not the emulsion tube from the main jet, but from the starter yet.

So what are the numbers of
main venturi size, aux venturi,
main jet 148 , main emulsion tube, main air corrector
idle jet 57, idle jet holder
accelerator jet number (horizontal or vertical)
Do the carbs have a label with a number?
 

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I am tuning H's as well, and found this. It is very interesting..


Type 1 the first DHLA40 (no letter after) if you like the DCOE of the DHLA family, but boasts superior atomisation and tuning ability to the extra research at Dellorto...This carb has an idle jet (which also runs the progression and adds mixture at cruising) that is not connected to the main jet stack, the idle jet operates on its own from the float bowl. Also the idle jet air feed for the emulsion tube is ot fixed at a set size like later models...The main jets run on their own from the float bowl like the idle's...This gives two seperate circuits that must be tuned for perfect overlap to get good cruise and pickup....This has the benefit on small engines and racing engines and hard tuned engines of giving the ability to make the idle jet progress long or short, you can bring the mains in early and cut back the progression on the idle jet air feed with differing jet holders with differing sized air holes, more air, bigger hole less progression....So you can tune this carb a million ways for million application, Fewsters just got a pair of these. They are the ulimate in tuneablity, they are also as a downside extremely hard to tune without a massive jet box or rolling road, cause they need to be bang on the money. This type of carb uses a .1 venturi which has a bigger pilot hole that the .2 and .3 (ill cover later), this means a bigger signal is generated in the venturi tube which gives a bigger signal (pulsing) to the main jet stack - the venturi feeds the main power circuit into the engine, the big jets. So the .1 vent can tuned to get on the mains right away from very low rpm and are super responsive to tuning, you can delay the vent by adjusting the tubes and air correctors on the main jet stack, so it needs more signal to start the stack feeding the vent or less setup the opposite way, so they can be tuned many ways also.

Type 2 DHLA40E - These are the same as the DHLA40 with seperate circuits and use either 1 or 2 venturi, they have extra progression holes for the idle jet, so the progression phase can be increased in length (over more plate angle) to suit production cars, and aid economy, real smooth runners when setup with lengthy progression. The 2 vent is just like a 1 with tiny bit smaller bore size to the signal tube, so they have fitted .2 often in the E model as they have extended the progression phase and using the .2 vent means they don't have to delay the mains as much as fitting the .1 vent, so making jetting easier and more responsive. The Elan +2 used these with the 1 vent as its punchy sports car, the Alfa's used 2 vent and fitted the E model to flagship models with lazyier owners, who like crisp but relaxed power delivery. Power is equal on both types and they can both delivery the same results.

Type 3 DHLA40F-G-H-N-R-S models. These are called emission completely different to the DCOE science, but noone really understands them and tries always to tune them like the early models. These differ because they have more progression holes and use the idle jet to feed most of the cruise phase and low rpm/low TP area of the engine AT ALL TIMES when the main jet isn't in operation completely automatically no jetting needed. The idle jet has a very large fixed 2.2mm air feed, you cannot tune this phase of the carburetor for length like the others, but here lays the secrets....

The idle jet doesn't feed from the float bowl, it feeds directly from the main jet stack, what happens after this is what gives these Dell'Ortos the sweetest road behaviour (what 25years of research came up with and not just for emissions) and dead easy tuning, you see when the main jet starts to emulsify fuel in the tube, the idle jet is feeding from it, so the gassy airy fuel shuts down the idle jet and sucks backwards yes BACKWARDS through the idle!! using every drip of fuel efficiently without ANY waste in circuit cross over where one is going after the other...this happens the moment the main jet fires, so there is NO need at all to tune the length of the progression and idle phase. This is pure magic they are automatically calibrated, you simply keep the idle jet above 59 up to 62 and not make the mistake of fitting numbers suited to the early DHLA or DCOE - with this simple technique you can tune anything from a 1300 to a 2000cc without really doing anything. The emulsion tubes in these carbs are always 8-10-11 and have to stay that way - which are really rich and have a hole straight down with loads of air holes, these atomise the fuel to an massive degree, also they have to be used cause the idle jet will not run correctly using the DHLA40-E type tubes (1-6-7-5) as the idle jet needs these airy tubes to function and cut out as designed...Usually people ram the DHLA40 idle holders with air holes in these carbs add the 1-6-5-7 style tubes and wonder why its a massive lean spot off idle, cause they missed the point completely!...These carbs use a .3 vent which has a very small signal tube to the main jet stack, this is because the holey tubes are basically ready to go from about 1250rpm (on my car using an 8tube 1 vent it was on the mains at 1250rpm!) so the holey tubes need holding back with a signal killing venturi...These carbs are wicked if you want bolt on power, they tune themselves!

The early types are better in respect of punch and tuneability on odd applications but dont' really do anything these late types won't on all but the best engines...I have run em all! These carbs will give the maximum power available on any engine if you take time to tune them right, but they are a bit more suited to standard motors over 1500cc, motors with mild cams or standard cams and they operate best if you are using 30-33 chokes, they hate race engines, mad cams and mad chokes cause they they are designed mainly for hot production engines with a clear pulse strength to suit the retarded venturi and tubes - great where silk town driving and alike is paramount and you do commuting or just sunday driving, they also give superior economy to the early types.

So the DCOE is the same as the DHLA40, but the DHLA40E and F-G-H-N-L-R-S are all evolutions of a principle and provided the application is matched to the carbs best qualities you have the ability to cater for everyones tastes and requirements using Dellorto...
 

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I am tuning H's as well, and found this. It is very interesting..


Type 1 the first DHLA40 (no letter after) if you like the DCOE of the DHLA family, but boasts superior atomisation and tuning ability to the extra research at Dellorto...This carb has an idle jet (which also runs the progression and adds mixture at cruising) that is not connected to the main jet stack, the idle jet operates on its own from the float bowl. Also the idle jet air feed for the emulsion tube is ot fixed at a set size like later models...The main jets run on their own from the float bowl like the idle's...This gives two seperate circuits that must be tuned for perfect overlap to get good cruise and pickup....This has the benefit on small engines and racing engines and hard tuned engines of giving the ability to make the idle jet progress long or short, you can bring the mains in early and cut back the progression on the idle jet air feed with differing jet holders with differing sized air holes, more air, bigger hole less progression....So you can tune this carb a million ways for million application, Fewsters just got a pair of these. They are the ulimate in tuneablity, they are also as a downside extremely hard to tune without a massive jet box or rolling road, cause they need to be bang on the money. This type of carb uses a .1 venturi which has a bigger pilot hole that the .2 and .3 (ill cover later), this means a bigger signal is generated in the venturi tube which gives a bigger signal (pulsing) to the main jet stack - the venturi feeds the main power circuit into the engine, the big jets. So the .1 vent can tuned to get on the mains right away from very low rpm and are super responsive to tuning, you can delay the vent by adjusting the tubes and air correctors on the main jet stack, so it needs more signal to start the stack feeding the vent or less setup the opposite way, so they can be tuned many ways also.

Type 2 DHLA40E - These are the same as the DHLA40 with seperate circuits and use either 1 or 2 venturi, they have extra progression holes for the idle jet, so the progression phase can be increased in length (over more plate angle) to suit production cars, and aid economy, real smooth runners when setup with lengthy progression. The 2 vent is just like a 1 with tiny bit smaller bore size to the signal tube, so they have fitted .2 often in the E model as they have extended the progression phase and using the .2 vent means they don't have to delay the mains as much as fitting the .1 vent, so making jetting easier and more responsive. The Elan +2 used these with the 1 vent as its punchy sports car, the Alfa's used 2 vent and fitted the E model to flagship models with lazyier owners, who like crisp but relaxed power delivery. Power is equal on both types and they can both delivery the same results.

Type 3 DHLA40F-G-H-N-R-S models. These are called emission completely different to the DCOE science, but noone really understands them and tries always to tune them like the early models. These differ because they have more progression holes and use the idle jet to feed most of the cruise phase and low rpm/low TP area of the engine AT ALL TIMES when the main jet isn't in operation completely automatically no jetting needed. The idle jet has a very large fixed 2.2mm air feed, you cannot tune this phase of the carburetor for length like the others, but here lays the secrets....

The idle jet doesn't feed from the float bowl, it feeds directly from the main jet stack, what happens after this is what gives these Dell'Ortos the sweetest road behaviour (what 25years of research came up with and not just for emissions) and dead easy tuning, you see when the main jet starts to emulsify fuel in the tube, the idle jet is feeding from it, so the gassy airy fuel shuts down the idle jet and sucks backwards yes BACKWARDS through the idle!! using every drip of fuel efficiently without ANY waste in circuit cross over where one is going after the other...this happens the moment the main jet fires, so there is NO need at all to tune the length of the progression and idle phase. This is pure magic they are automatically calibrated, you simply keep the idle jet above 59 up to 62 and not make the mistake of fitting numbers suited to the early DHLA or DCOE - with this simple technique you can tune anything from a 1300 to a 2000cc without really doing anything. The emulsion tubes in these carbs are always 8-10-11 and have to stay that way - which are really rich and have a hole straight down with loads of air holes, these atomise the fuel to an massive degree, also they have to be used cause the idle jet will not run correctly using the DHLA40-E type tubes (1-6-7-5) as the idle jet needs these airy tubes to function and cut out as designed...Usually people ram the DHLA40 idle holders with air holes in these carbs add the 1-6-5-7 style tubes and wonder why its a massive lean spot off idle, cause they missed the point completely!...These carbs use a .3 vent which has a very small signal tube to the main jet stack, this is because the holey tubes are basically ready to go from about 1250rpm (on my car using an 8tube 1 vent it was on the mains at 1250rpm!) so the holey tubes need holding back with a signal killing venturi...These carbs are wicked if you want bolt on power, they tune themselves!

The early types are better in respect of punch and tuneability on odd applications but dont' really do anything these late types won't on all but the best engines...I have run em all! These carbs will give the maximum power available on any engine if you take time to tune them right, but they are a bit more suited to standard motors over 1500cc, motors with mild cams or standard cams and they operate best if you are using 30-33 chokes, they hate race engines, mad cams and mad chokes cause they they are designed mainly for hot production engines with a clear pulse strength to suit the retarded venturi and tubes - great where silk town driving and alike is paramount and you do commuting or just sunday driving, they also give superior economy to the early types.

So the DCOE is the same as the DHLA40, but the DHLA40E and F-G-H-N-L-R-S are all evolutions of a principle and provided the application is matched to the carbs best qualities you have the ability to cater for everyones tastes and requirements using Dellorto...
I came across to your message and I was really impressed! However I need your advice on my setup as I run on a common(?) problem using DHLA 40H on my stock Nord engine 2.0 L which actually came from a motronic A75 or Alfetta GTV engine (AR01713). This engine runs with 10.1:1 CR and uses 10548xxxxxxncamshafts (which were stock for these engines in their carbirated engine). The problem is a typical(?) hesitation around 1500 RPM, beyond that the engine revs amazingly. As far as I understand all jets on my DHLA 40H are stock and the idle is 0.58, so I think to replace idles with 0.60. What do you think? will be ok? what else need to take care? Ignition is electronic with 123 distributor and advance curve identical to original Bosch distrubutor fitted before.
Any comments will be welcomed! Thanks in advance.
 

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Richard Jemison
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Install a Wide Band O2 meter so you know what your AFRs are at all engine speeds and loads. Then you can adjust jetting to get it right.
 

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But Mad North-Northwest
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So you've got emissions Dellortos. This isn't a bad thing, they're a more sophisticated design than the older style, but they need to be jetted differently. They need specific emulsion tubes (8, 10, or 11) and they need a specific idle jet holder without an air calibration hole (the top is solid, there's no inlet where the screwdriver slot is). If you use the wrong tubes or the wrong idle jet holders you will apparently get a big lean spot off idle.

The book jetting I can find for a DHLA H in an Alfetta 2.0 engine is below. I'd start by double-checking that you have all the right jets & emulsion tubes, and also the right idle jet holders.

These are the holders you need:


1680086

1680088
 

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But Mad North-Northwest
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Here's an explanation I found on how the emissions DHLAs differ and the jetting requirements. I have a set of these on my GTV with stock Alfa jetting and they work absolutely great.

Type 3 DHLA40F-G-H-N-R-S models. These are called emission completely different to the DCOE science, but noone really understands them and tries always to tune them like the early models. These differ because they have more progression holes and use the idle jet to feed most of the cruise phase and low rpm/low TP area of the engine AT ALL TIMES when the main jet isn't in operation completely automatically no jetting needed. The idle jet has a very large fixed 2.2mm air feed, you cannot tune this phase of the carburetor for length like the others, but here lays the secrets....

The idle jet doesn't feed from the float bowl, it feeds directly from the main jet stack, what happens after this is what gives these Dell'Ortos the sweetest road behaviour (what 25 years of research came up with and not just for emissions) and dead easy tuning, you see when the main jet starts to emulsify fuel in the tube, the idle jet is feeding from it, so the gassy airy fuel shuts down the idle jet and sucks backwards yes BACKWARDS through the idle!! using every drip of fuel efficiently without ANY waste in circuit cross over where one is going after the other...this happens the moment the main jet fires, so there is NO need at all to tune the length of the progression and idle phase. This is pure magic they are automatically calibrated, you simply keep the idle jet above 59 up to 62 and not make the mistake of fitting numbers suited to the early DHLA or DCOE - with this simple technique you can tune anything from a 1300 to a 2000cc without really doing anything. The emulsion tubes in these carbs are always 8-10-11 and have to stay that way - which are really rich and have a hole straight down with loads of air holes, these atomise the fuel to an massive degree, also they have to be used cause the idle jet will not run correctly using the DHLA40-E type tubes (1-6-7-5) as the idle jet needs these airy tubes to function and cut out as designed...Usually people ram the DHLA40 idle holders with air holes in these carbs add the 1-6-5-7 style tubes and wonder why its a massive lean spot off idle, cause they missed the point completely!...These carbs use a .3 vent which has a very small signal tube to the main jet stack, this is because the holey tubes are basically ready to go from about 1250rpm (on my car using an 8tube 1 vent it was on the mains at 1250rpm!) so the holey tubes need holding back with a signal killing venturi...These carbs are wicked if you want bolt on power, they tune themselves!
 

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Here's an explanation I found on how the emissions DHLAs differ and the jetting requirements. I have a set of these on my GTV with stock Alfa jetting and they work absolutely great.

Type 3 DHLA40F-G-H-N-R-S models. These are called emission completely different to the DCOE science, but noone really understands them and tries always to tune them like the early models. These differ because they have more progression holes and use the idle jet to feed most of the cruise phase and low rpm/low TP area of the engine AT ALL TIMES when the main jet isn't in operation completely automatically no jetting needed. The idle jet has a very large fixed 2.2mm air feed, you cannot tune this phase of the carburetor for length like the others, but here lays the secrets....

The idle jet doesn't feed from the float bowl, it feeds directly from the main jet stack, what happens after this is what gives these Dell'Ortos the sweetest road behaviour (what 25 years of research came up with and not just for emissions) and dead easy tuning, you see when the main jet starts to emulsify fuel in the tube, the idle jet is feeding from it, so the gassy airy fuel shuts down the idle jet and sucks backwards yes BACKWARDS through the idle!! using every drip of fuel efficiently without ANY waste in circuit cross over where one is going after the other...this happens the moment the main jet fires, so there is NO need at all to tune the length of the progression and idle phase. This is pure magic they are automatically calibrated, you simply keep the idle jet above 59 up to 62 and not make the mistake of fitting numbers suited to the early DHLA or DCOE - with this simple technique you can tune anything from a 1300 to a 2000cc without really doing anything. The emulsion tubes in these carbs are always 8-10-11 and have to stay that way - which are really rich and have a hole straight down with loads of air holes, these atomise the fuel to an massive degree, also they have to be used cause the idle jet will not run correctly using the DHLA40-E type tubes (1-6-7-5) as the idle jet needs these airy tubes to function and cut out as designed...Usually people ram the DHLA40 idle holders with air holes in these carbs add the 1-6-5-7 style tubes and wonder why its a massive lean spot off idle, cause they missed the point completely!...These carbs use a .3 vent which has a very small signal tube to the main jet stack, this is because the holey tubes are basically ready to go from about 1250rpm (on my car using an 8tube 1 vent it was on the mains at 1250rpm!) so the holey tubes need holding back with a signal killing venturi...These carbs are wicked if you want bolt on power, they tune themselves!
Hi Gubi! The picture below is from my books for a Nord 2.0 L and in particular for AR01655. As I said my motronic AR01713 has been "downgraded" to AR01655 including the correct cams 10548xxxxxx which I fitted just yesterday, so far the engine was running with the cams designated to the injection setup... My carbs setup is slightly different than my book in emulsion tube which makes a slight mismatch... The idle jet holder is blinded, not exactly as per your sample but definitely is blind. So far, as I can see my setup is according to your book. Certainly all this is deep waters for me and the only I want is to eliminate the frustrating dead spot in 1500 rpm area. Tomorrow I will see for review and possible purchase a pair of 40H took out from a donor car which alleged to be fully stock. The idea to increase slightly the idle jet size came from other relevant posts which make some sense to me as the whole matter seem to be a fuel starvation. Also I will recheck my float adjustment, before was 15mm but now is 16mm further to a recent change I did following suggestions from a Dellorto guru...
 

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But Mad North-Northwest
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Okay, which emulsion tube do you have? Needs to be an .8, .10, or .11 with those carbs.

Can't hurt to try a fatter idle jet but I doubt that's your problem. I've got an Alfetta 2L in my GTV with 10548 cams and DHLA 40G carbs and it's running perfectly on 58s. Assuming the jetting is right you should probably do what Richard said and have it tested with a wideband O2 sensor, otherwise you're just guessing what the problem is.

You should also verify fuel pressure, don't want more than 3 PSI or the pumps can overwhelm the needle valves at idle and you can get a stumble off the line.
 

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All they are .11, some times the problem is not noticeable.... however it's very obvious if I am in 2nd gear with very slow speed let's say 10km/h and try to accelerate. In normal acceleration starting on a traffic light is almost unnoticeable. It drives me crazy, I have tried very much with idle mixture, I am not running lean nor rich either....carbs were serviced, cleaned, perform fantastic, no rattling or shaking. Idle in 850 , no vibrations.... Fuel pressure between 3-5 psi , BUT something is wrong...
 
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