Dellorto DHLA40G - questions/opinions!

[jono]

I'm refurbishing these carbs which, as I understand it, are an emissions version.

Interestingly they came from a 1979 Lotus twincam tall block engine which was built 'brand new' by Vegatune for a Caterham silver jubilee seven (not a belt drive Vegantune, a traditional Lotus TC head).

I was expecting to find 5 progression holes but have discovered the the final two holes on each barrel have been soldered up thereby leaving three as per the early type DHLA40. They have 32mm chokes

What would be the advantage of this (if any) - an attempt to turn the only available new DHLA's at the time into a tunable performance carb?

What would be the effect of such a mod - to bring the main jets in sooner? I ask because I am going to try these carbs on my plus 2 which has always suffered seemingly unsolvable transition stumbling on the existing three hole DHLA40's (with 33mm chokes).

I am considering drilling/melting out the solder and restoring them to 5 progression holes.

Any wisdom - leave as is because Vegatune would have known what they were doing and they are already jetted for a Lotus TC, ..or revert to 5 holes?

I am mindful that a seven is much light than a Plus 2 so the jetting is probably not optimised anyway for the heavier car but I figure its worth a punt for the cost of a rebuild kit.

[Verve]

Hi Jono ,

I would be wary with these emission carb`s ... the Des Hammill book ( 3rd edition page 68 ) does not recommend them for tuning ... even though there are ways around it ... which no doubt Vegantune did ... but like you say , for a much lighter car with more torque from a tall block ...

Also on Page 9 , he states " Any Carburettor which has 2 or 3 very small diameter progression holes will have a rich progression phase ... whilst Carb`s with 5 quite large progression holes will have a weaker progression phase ... "

Not being an Engineer , I`ve never really understood the science behind that ( maybe somebody can explain it to me ; ) ??

However , Des Hammill totally recommends the "Universal Performance" carb`s for tuning , recognised by only having the 3 progression holes , as they give the best " all round accelerative & top end performance " .

For what it`s worth , with the correct carb`s for the twincam ... we`ve always been able to dial out the off idle stumble/hesitation , by simply ( as you should ) increasing the richness of the "Progression Jet" ( idle jet ) ... assuming all other things are correct of course , including float levels , even ignition timing & finally fuel used : )

Hope this helps ,

Cheers ,

Paul .

[RichardHawkins]

Jono,

I suggest you join sidedraftcentral a web site run by a gentleman in Calafornia called Keith Franck. Keith has been doing a load of work for years. He is a retired physicist and would seem to know more about these carburettors than anyone else, and seems to have endless patience with user questions.

Richard Hawkins

[types26/36]

Jono,
The emission carbs are a "black art" and it is very hard to get any info on how they work as they work in a different way to non emission carbs.
The idle jet is not fed from the float chamber as per non emission carbs but it is fed from the main jet stack and the carbs have a fixed 2.2mm air feed which you cannot change.
Do not mix up the idle jet holders with non Emission holders as they are different.
I ran my Sprint with 40G's for a couple of years and although the car was running OK the only way was to jet them very rich, the plugs and exhaust were always black (it was in the days when you could still get leaded fuel)
At the time I did a bit of research on them and found a posting on a forum that explained the workings (to a certain extent)
I can email you the post (in word format) which should help you to understand the workings. (pm's do not allow word) so would need your email address.
If your 40G's have been modified by Vegantune then I would first try them before removing the solder from the progression holes ........you can always do that at a later stage.

[mark030358]

Brian, You are a constant source of information

[jono]

Many thanks for all you suggestions guys - much to mull over there!

I think I will rebuild the carbs and try them as at least that will provide a comparison with the non emissions set.

My plan was always to go fuel injection (I'm half way there with a FI tank, twin pipes and wasted spark ignition) but everytime I reach the cusp of doing it I look at the cost and something else calls on my hard earned!

Will keep you posted, thanks again

[Verve]

Hi Jono ,

My new Elan has QED Fuel Injection ... the weather is not helpful to the experience at the moment ... but next time you are over this way , you are welcome to have a drive : )

Cheers ,

Paul .

[jono]

Hi Paul,

That's really interesting - what are your experiences before and after and how does it drive compared to a carb'd Elan?

I'm some of the way there already as when I built my car I installed a Andy Wiltshire FI tank and all of the flow and return pipework and I already have the crank triger system and TPS as I run a wasted spark ignition system. The intention was alway to go to FI but at the moment I feel like selling all of my classic cars and buying an Evora as I'm falling out of love with working on them! (a pal has an Evora 400 and it's simply stunning in every way)

Cheers

Jon

[Verve]

Hi Jon ,

Yeh , we all feel like that sometimes ... just close the garage door for a few days ... it will soon pass

I`m lucky enough , that after a lot of fettling my standard Elan S4 on webers drives like it is on Fuel Injection ... however I will know for sure , once the weather has improved so I can drive my new S3 with the injection properly ....

So far , the short test drive from cold was impressive ... it just purred

I will keep you posted ,

Cheers ,

Paul .

[jono]

Looks lovely Paul, ....are Mike's fingerprints on that per chance?

....will look you up when next at the inlaws

[Verve]

Thanks Jon ,

Mike hasn`t seen it yet ... but hopefully will do soon

Yeh , just let me know when you are in the area ...

Cheers ,

Paul .