Lancia Beta Forum

Technical stuff => Fuel System Carb => Topic started by: capriblu on January 09, 2021, 09:19:06 PM

Title: Jetting on standard DATR/DAT type Carb
Post by: capriblu on January 09, 2021, 09:19:06 PM
My Car (2000 Coupe) generally runs very smoothly but I have struggled to get idle quite as smooth and stable as I always remember being to achieve with these cars many years ago.  I have a couple of different DAT /DATR 250 variant carbs that I have tried and all are similar.  I'm confident that ignition timing, no air leaks, spark etc. is all good and I'm starting to wonder whether changes in fuel quality over the years have started to make the original jetting a bit marginal?

Has anybody got any experience or thougts on this?

I'm tempted to try some slightly larger size idle jets.  

Presume both the primary and secondary idle jets will affect tickover (as well as low/medium speed and load running)?

What are the standard jet sizes?

I notice in the Haynes Manual (never a great thing to rely on!) that different main air correction jets on the secondary are shown for DATR variants with 1.50/2.10 and 1.5/2.40 (main/air corrector) the former for 1600 and the latter for some 2000 cars.  This seems quite a difference?

Also quotes a 1.00 mm idling jet on secondary for earlier DMTR (1600/1800) v 0.8 on the DATR both for the same 0.70mm air jet size?

Any sage advice welcome .......... ???  

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: peteracs on January 09, 2021, 10:04:03 PM

Out of interest, what petrol are you using?

To rule out the ethanol influence you could try the Esso 99 which according to them has no ethanol in it.

Also a few experiences with dirty idle jets needing a good clean.

Also any chance the float is not floating as it used to?

There was also on the LMC forum an older carb where the end of the idle metering needle had become deformed and needed to be reformed.

Just my 2 penny worth...


Title: Re: Jetting on standard DATR/DAT type Carb
Post by: capriblu on January 09, 2021, 10:28:13 PM
Thanks Peter,

Issue is very minor really but I know all fuel is very different now.  Ive also read that the Esso 99 is ethanol free so i may try that. I generally use Shell Optimax.

I think the settings originally were never super generous in the lower speed / load range.  The jets are clean and the mixture needle is good with a new rubber o-ring seal.  Ive checked and re-set the floats a while ago but this shouldnt really affect idle or low load. I think I'm just going to try a few different idle jet sizes in 0.05mm increments and see if I notice anything .......  

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: mangocrazy on January 12, 2021, 05:29:50 PM
I have no Lancia-related information, but I do know that carburetted motorcycles from the late 80s benefit from larger idle/pilot jets and probably also main jets. My 1988 VFR750 was much more responsive when I went up one size on the pilot jets.

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: smithymc on January 12, 2021, 08:02:24 PM
My primary stage was increased by the Aldon  guys following the rolling road session. Now if I can just find the paperwork........


Title: Re: Jetting on standard DATR/DAT type Carb
Post by: JohnFol on January 13, 2021, 05:00:44 PM
I've something in the back of my mind that bigger jets are the way forward as petrol is more "diluted" that when the cars came off the production line. Increased jets compensates for this. however .. . . I can't find the original source of this so anecdotal but interested if anyone else remembers the article

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: capriblu on March 07, 2021, 08:38:07 PM

So with 20 litres of fresh Shell Optimax with a splash of Millers VSP I set about trialling different jet and air corrector cap sizes. 

Before my new items arrived I had already tried running the car with a spare standard 0.8 secondary idle jet on the primary side.  This made a noticeable difference to pick-up and general feeling of responsiveness from low engine speeds but obviously a bit of a big jump from the 0.5 standard jet size.

I tried the same experiment with a spare carb (both DAT 2/250s) and found exactly the same behaviour.  As well as the general difference in feeling of responsiveness when actually driving then, again for both carbs sudden blipping the throttle at rest with a standard 0.5 idle jet (I tried two different 0.5 jets just to be sure) seemed to be met with a very slight but nonetheless discernable fluff/miss.  This disappeared with the 0.8 jet.  (Ignition fine and timing checked).  I was still able to keep CO at idle reasonably steady at around 2.5/3.0%.

So my new jets and air corrector caps arrived.  (About 30 worth from Eurocarb).

Firstly I tried just changing the primary idle jet.  A 0.6 made a very big difference over the standard 0.5 but car still didn't seem to pick up just quite as well as a 0.8.   Then tried a 0.7 and this felt good. Perhaps a 0.65 would be optimal.

Subsequently took a punt with a selection of the few items that I purchased and went for this combo -

Primary Idle   0.7 instead of 0.5 standard
Primary  Main  130 instead of 120 standard,  kept standard F30 Emulsion Tube,  Air Corrector Cap 160 instead of 170
Secondary Idle 1.0 instead of 0.8 standard  (doesn't seem to be anything available between 0.8 and 1.0)
Secondary Main 155 instead of 140 standard, kept standard F30 Emulsion Tube,  Air Corrector Cap 225 instead of 240

With the above the car is absolutely transformed.  Absolutely no wooliness around transition from first to second choke and just seems smooth and strong throughout engine speed range with only a slight second choke kick which just suggests better power lower down rather than any loss at the top. 

A few short speed runs on some familiar stretches confirmed that the car is noticeably swifter and seemingly freer to head towards redline.

I could "back-off" from this combo slightly to see at what point performance/response starts to fall but for the time being I'll stick with this and see how the car performs on longer runs.  Hopefully MPG will still be sensible.  I checked the plugs after a 30 minute run and all nice and light brown.

I always felt the fuelling on the 2 litre cars to be marginal and this along with the advent of current ethanol containing pump fuels probably means the original standard jetting is now seriously inadequate.

Really pleased - car is just so much better and quicker!

I had an early F/L  HPE 2000 as a daily over 25 years ago and this drove similarly.  All the other 2000 Betas that I have driven have suffered from very slight first to second choke "fluff"to some extent. This effect could often be eliminated by retarding ignition very very slightly but at the loss of top-end performance.  Shortly after I bought my HPE Chris Bastow/Betacar rebuilt the engine with HC pistons - I'm wondering if the Carb was re-jetted as well!

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: mangocrazy on March 07, 2021, 11:14:33 PM
Excellent stuff! I will at some point be replacing the standard Weber on my Spider 2000 with a pair of DCOE 45s with 36mm choke, but until that happy day I think that uprating the jets as you have done will make the car much nicer to drive. Where did you source the jets from?

<Edit> - Just re-read your post and noticed you used Eurocarb. I'll speak to them in the morning.

Title: Re: Jetting on standard DATR/DAT type Carb
Post by: JohnFol on March 23, 2021, 10:02:18 AM
Just found this useful calculator.  Also has good descriptions of what the effect is of increasing or decreasing values (ie smaller choke) (