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Discussion Starter #1
I am currently dialing in a set of lightly used "H" carbs, which are new to me, and have found some interesting info that is specific to emissions carbs. While tuning my freshly rebuilt 2.0, I couldn't shake a consistently lean condition below 3k rpm and at cruising, based on all explanations. (My wideband isn't installed yet, waiting for the weather to cooperate.) and I have seen at least one other recent post, where someone else could really use the info..

In researching, to get a better understanding of the emissions carbs vs. the earlier versions, I ran into a re-posting of a description of all of the DHLA incarnations.

the jetting config that came in my "H" carbs is as follows:

air 200
emulsion .7
jet 35

H type corrector
jet 55

jet 33

Venturi .3

Here is the article of interest, and for the emissions carb folks, pay close attention to "Type 3"...

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...

Grabbed from : Dellorto DHLA 40 universal performance

3 Posts
Discussion Starter #2
to follow up on my jetting issue, based on the needs of these carbs.

I will be base-lining the carbs, installing the correct tubes and jet sizes. I will probably go tho the richer end, based on the fact that my motor is a warmed up 2.0l with added cam duration and compression. Ignition is set at full 34 degree advance @ 2500 rpm.

.11 emulsion tube
.60-65 main jet
.62 idle jet

I will report back.

485 Posts
Sorry, what is 65 main jet? 0.85 seems to be the smallest size they offer.
For 2 liter engines, 1.40-1.50 sizes look reasonable.

Here is my current setup:
34 & .3 Venturi
11 tube
1.50 main jet
2.10 air
.57 idle jet (may be replaced by .59 or .60 later)
idle screws are 4.5 turns off
.40 acc. jet
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