Lotus Elan and Lotus Elan Plus 2 Forums

My rebuilt Elan S4 SE spec Twin Cam is fitted with Dellorto DHLA40E (the European version) carburettors. Can anyone advise me on what jet sizes and emulsion tubes I should fit as a starting point?

Is the engine still stock, or did you have it hot-rodded a bit during the rebuild? The engine's state of tune has a direct impact on the required carb jetting. Carbs, cams & compression should always be kept in balance.

If the engine is stock, then use the same chokes, emulsion tubes and jets as the DHLA 40 set-up that appears in the manual. The '40E' carbs have revised progression hole drillings that are not intended to be user adjustable, and the idle mixture screws have a finer thread and a longer taper to the needle. During tuning, the idle mixture screws will end up more turns out from seated (3 - 3 3/4) than would the screws in a standard '40' carb.

But the basic starting point set-up would be the same as given in the manual, including the fuel pressure. If necessary, install an inline fuel pressure regulator as close to the carb inlets as possible. The fuel pressure should be 1.5 - 2.5 psi at the carb inlets.

What floats are installed in the carbs? The weight and height setting are fundamental to the carb's mixture control.
DHLA 40 = 10 gram Float (7298-01) set to 14.5 to 15 mm
DHLA 40E = 8.5 gram Float (7298-02) set to 16.5 to 17 mm

Each float weight has a companion height setting that's required to produce the required fuel level in the bowl. If your 40E carbs don't have 8.5 gram floats, then set the height to the appropriate level for the floats that are being used.

What you want is a fuel level of 27mm (1.063") measured from the top of the jet pedestal down to the surface of the fuel pool. A smaller dimension equals a higher fuel level and richer overall mixture... and vice versa. Run the engine long enough to ensure the fuel pump has fully filled the bowl. Switch off, promptly remove the carb's top cover, and use a depth gauge to measure the fuel height. Adjust the float height as required, remove a little of the fuel so the float starts from a low position, and repeat the test.

The 'E' carbs (Emissions) tended to be set a little too lean for good running from the factory, so you may have to go a step or two richer on both the Idle Jet and the Idle Air Corrector. However, the best running condition may result in emissions rising above allowed limits. Back in the day, you lived with an emissions-lean mixture if your engine was subject to emissions tests. Your call.

With the engine warmed up, the carbs accurately balanced (that's critical), and the Idle Mixture Screws adjusted for a smooth idle, crack the throttle open slowly enough that the Accelerator Pump isn't much of a factor. If the engine hesitates off idle, then go a step or two richer on the Idle Air Correctors. If the engine does not hesitate, then go leaner until the engine just picks up a hesitation, then go one step richer. The idea is to be as rich as necessary to avoid a hesitation, but no richer.

Go for a drive, and accelerate at full throttle in a middle gear (depends upon the road you have available). If the engine stumbles at about 3200 rpm, then go a step or two larger on the Idle Jets. If it doesn't stumble, then go smaller on the idle jet until the engine just stumbles, then go one step richer... or as required for the stumble to just disappear.

The Idle Jet and Air Corrector work as a team, and they affect one another; so adjusting one may require a companion change to the other. Repeat the above process until the last round produces no further change.

Regards,
Tim Engel

Hi Tim, thank you very much for your comprehensive response. I will follow your instructions. I happen to notice in Miles Wilkins Twin Cam book that on page 187 he lists comparison jet sizes between the 40 and 40E carbs. For the SE spec engine fitted with DHLA40E carbs it lists a 120 main jet, a 200 air corrector jet, 45L slow running jet, 7850-4 Idle Jet holder, 7772-5 Emulsion Tube and a 33 pump jet. Are these sizes specified for European emissions only and therefore should I follow the standard SE sizes as you suggest? In particular the 200 air corrector jet seems quite different to the 160 DHLA size. Regards, William

Emissions standards were coming into play back then, with some countries jumping on the bandwagon earlier than others. The really stringent countries got Zenith-Stromberg carbs on their Twin Cams. But even in those markets served by Dellortos, there were non-emissions and emissions set-ups for the Dellortos. The older Weber DCOE set-ups weren't even in the emissions game.

One thing is for sure, the emissions set-ups ran leaner, and made low emissions more of a priority than either driveability or power. Even after the Weber DCOE was dropped, the UK domestic market cars kept the most sporting/ higher emissions/ richer set-up. The European ECE-15 countries required lower emissions, so they got the DHLA 40 'E' carbs, and a leaner set-up. As you might suspect, the engines didn't run as well (in a sporting sense) as the UK domestic versions. At the time, the Federal emissions standards were the most strict, they got the Zenith-Stromberg carbs, and a version of the Twin Cam that was way down on power compared to any Weber/Dellorto tuned version.

So, where do you want your engine to be? Choose, since, "Both" isn't an answer.

"IF" you live in an area where your vintage car must pass an annual emissions test, then you should choose a carb set-up that is at least as clean as the one that originally came on your car. On the other hand, if the engine doesn't have to meet any emissions standard, then the more sporting UK domestic set-up will make more power... and burn more gas.

In Wilkins' book, pg 187, the two columns for the UK domestic engines call for higher float settings (richer), while the European columns call for lower float settings (leaner). The fuel level is fundamental to setting the rich-lean tone for the carbs. The important spec that is missing from Wilkins' chart is the float weight. Depending upon the period, there were 3 or 4 different float weights available as tuning tools, and the float height is rather meaningless if you don't also know the weight. A lighter float set deeper (16.5-17.0 mm) can produce a fuel level that is about the same as a heavier float set higher (14.5-15.0 mm). Lotus was prone to setting the float height higher (richer) than the Dellorto guideline in order to squeeze the last horsepower out of the engine. At the moment, I don't have time to look into which weight float Lotus used for each set-up. I'll look into it later this evening.

But taken at face value (not knowing the float weight), in Wilkins' first column, the 30 main jet is doing it's thing in an overall richer environment (high float setting), and in the second column, the same 30 jet is working in an overall leaner environment. The end result is not the same. The same applies for the other component sizes. You can't just compare numbers, you have to evaluate the overall combination, not just the size numbers in one row.

Columns 1 & 2, UK Domestic, are more biased toward sporting performance. Columns 3 & 4, European, are more biased toward cleaner emissions and economy. Again, where do you want to be?

Regards,
Tim Engel

The following agrees with Wilkins' book, pg 187, and adds a few missing lines.

Lotus Technical Service Bulletin ............................... CLASS: ...... 111
................................................................................... NUMBER: .. 1974/1
Title: ....... Workshop Manual Amendments. .............. TYPE: ........ All Models
Reason:.. To correct Technical Data as indicated. .... Date: ......... 12-Feb-1974
Action: .... Amend Dellorto carburetors settings, adding where necessary, as follows:
.................................... ------ Small Valve ------ .......... ------- Big Valve -------
.................................... UK .............. Euro ECE-15 .... UK .............. Euro ECE-15
.................................... Domestic .... Emission ........... Domestic .... Emission
Choke ......................... 30 mm ........ 30 mm .............. 33 mm ........ 32 mm
Main Jet ..................... 115 ............. 120 .................... 120 ............ 120
Main Air Correct Jet ... 160 ............. 200 .................... 130 ............ 160
Idling (Slow Run) Jet ... 50.02 ......... 45L .................... 50 .............. 50L
Pump Jet ..................... 40 .............. 33 ...................... 45* ............. 33
........................... if necessary to improve driving, Use 35*
Starter Jet .................... 70 .............. 70 ..................... 70 ............... 70
Main Emulsion Tube .... 7772-l ....... 7772-5 ............... 7772-5 ........ 7772-5
Starter Emulsion Tube.. 7482.1 ...... 7482-1 ............... 7482-1 ........ 7482-1.28
Needle Valve ............... 150.33 ...... 150.33 ................ 150.33 ........ 150.33
Idle Jet Holder ............. 7850-2 ...... 7850-4 ................ 7850-2 ........ 7850-1
.................................... (120) ......... (160) ................... (120) ........... (140)
Float Assembly (P/N) ... 7298-01 .... 7298-02 .............. 7298-01 ...... 7298-02
Float Weight ... (grams) 10 ............. 8.5 ...................... 10 ................ 8.5
Float Height .... (mm) ... 14.5-15.0 .. 16.5-17.0 ............ 14.5-15.0 .... 16.5-17.0
Air Trumpet Length ...... 40 mm ...... 40 mm ................. 40 mm ......... 40 mm
Colour Code ................ Green ....... Green .................. Red ............. Red
Part No. prefix - Front... N ............... P ......................... Q ................. R
Part No. prefix - Rear ... N ............... P ......................... Q ................. R
Idle Speed (rpm) ......... 900-950 .... 900-950 .............. 900-950 ....... 900-950
Cam ............................. C-Cam ...... C-Cam ................ D-Cam ......... D-Cam

Note that the float weights are different between UK domestic and Euro set-ups, as is appropriate for the float heights Wilkins gives on page 187 of his book. He omitted the necessary companion piece of information.

As time passes, some Dellorto parts are becoming harder to find, and I don't think all the float weights are still available. If availability forces you to change to some float weight other than what the Lotus spec calls for, then it's also necessary to change the float height accordingly. Maintaining the correct fluid level in the float bowl is what's important!

Regards,
Tim Engel

Many thanks for your help Tim. You are certainly a fountain of all knowledge!