DHLA 45 Jetting...

[shaunroche]

So I have a pair of Dellorto 45 dhla's fitted along with a Newman cam....how do I determine what Jetting is required?

Is it all purely down to running it on a rolling road?

[alolympic]

I rebuilt my pair with all new gaskets, jets etc. I sought advice from Webcon about which jets, but ultimately, it only got them running.
I went to Tom Airey down in Hampshire to get it tuned with the help of his rolling road. He replaced most of the jets and tubes with different ones. We started with 90bhp and I left with a lot more.
Based on my experience, it definitely feels like each engine will need the set up of an old school expert to get the right combo.

[Tony Burd]

Tom Airey did mine aswell, so much smoother after he worked his magic.

[zombeh]

Lotus spec 9 might be a good place to start, what are the carbs originally from/what's in them now?

Carb Type ...................... DHLA 45E
Choke .............................. 37 mm
Main Jet .......................... 160
Main Air Corrector Jet ... 230
Main Emulsion Tube ...... 7772-8
Idle Jet ............................. 55 (maybe a little lean, try up to 58)
Idle Jet Holder ................. 7850-9
Float Weight .................... 8.5 gr
Float Setting Height ........ 14.5 - 15.0 mm
Float Needle Valve ......... 170
Pump Jet .......................... 50H
Starter Jet ......................... 95
Starter Emulsion Tube ..... 7482-3
Power Jet .......................... ---
Slow Running Speed ....... 900 - 1000 rpm
Idle CO Level (hot) ........... 0.7 - 1.5%

[gmsclassics]

Yes, agree, when I fitted mine my 'old school expert' started with the standard jetting Lotus used for the Excel 2.2L. My engine has the STR139 cam and all he had to do was increase the size of the mid-range jetting which was otherwise a bit flat. Did that just by drilling out the jet so I can't tell what it is equivalent to. He just drove it up and down the road to be able to tell what it needed. Not had to change in years.

Geoff

[dursley92]

I got these recommendations from Eurocarb who are the Dellorto agents when I asked about a setup place to start from to give an engine with good low down grunt. Can't say how good it is because I havn't finished building the engine yet!

36 chokes
142 mains
180 airs
7772.5 emulsions
60 idles
7850.1 idle holders
42 pumps.

[shaunroche]

Hi chaps, thanks for the replies!

So asking again.....what determines what jets etc. are required, what is measured to give the correct jets? Is it down to emissions being measured or something?

[MIG Wielder]

I can also only comment from many years experience of the Lotus Elite 907 again with the Spec; 9 parts.
My current set up which has changed over the years most possibly due to engine wear is exactly as Zombeh quotes except for the following.
Idle jet is a 60
Idle jet holder is a 7850-7.
Main jets are 160.
My base-line set up is a complete clean of the carbs and set the float height.
(It doesn't change much.)
Then set idle mixture (hot) with a colourtune and suck-meter on each inlet.
This is a good basis for an MOT pass.
The next set up was ...how does it accelerate from idle ?
I think the next bit used to be called the "progression jetting" ,but it is really the idle jet setting a baseline fuel delivery into the cylinders and as you go up the rev; range under power the extra fuel is provided by the main jet. So I went with on-road acceleration tests increasing the idle mixture until I reached the end stop on all 4 idle screws.
Now originally I had 55L idle jets ( the "L" is a designation of a long jet but makes is little different to the 55 spec; jet ).
With new 60 jets I colourtuned it again ( the adjuster screws were much further in, ) and the progression was much better. It accelerated much better. I then changed the main jets from 150 to 160 and that felt right. After a long run the plugs look nice and light chocolate brown. And that is how it has been for some time now.
Yes, I could probably get it rolling road tested, but it passes the MOT and it feels O.K.
Now the Lotus is a twin cam 16 V version so the breathing is probably going to be rather better than a Sprint set-up.
So this is how I do mine. Yours won't be vastly different.
I think the important thing is to start with cleaned carbs with float height set.
If you get one mixture screw setting at idle very different to another, then its time for a re-think.
Then work from there.
HTH.
Tony.

[shaunroche]

Nice one Tony, cheers....I'll pass this on to the rolling road guy where I'm taking it soon and see what happens!