Dellorto Choke & Venturi Sizes

[William2]

I am rebuilding a secondhand pair of Dellorto 40DHLA carburettors to put on a Sprint spec engine. The carbs are currently fitted with 32mm Chokes and 7848-3 Auxiliary Venturis. Does it matter much that they are not exact sizes for Sprint i.e. 33mm and 7848-1? I did not want to spend more money on the carbs than I need to. I have fitted all new jets, etc.

[Esprit2]

Carb set-up is like a spider web. Tickle it here, it tingles over there. Everything is interdependent.

The choke size basically sets the baseline from which all other jetting and adjustments are made. You can make non-Lotus choke sizes work, but you will have to do all the related jetting yourself. If you were hoping to take all the standard Lotus Elan Sprint set-up bits and pieces except use non-standard chokes, and have it all work the same... or at least, work 'well'... that's not an option.

If you go with larger chokes (within reason), the carbs will run leaner overall (ie, weaker verturi vacuum won't pull the fuel over as well), so re-jet accordingly. Low end torque and throttle response will be weaker, but the engine will pull better at full throttle high rpm. It will be less pleasant in low speed traffic, but great at full blat on a country road.

If you go with smaller chokes (as you noted), the carbs will run richer overall (everything else being equal). If you re-jet everything else to work the best with the smaller chokes, then the engine will be more responsive off idle, accelerate off the line better, and be easier to drive in traffic, but top end horsepower will drop off and the engine will run out of breath earlier in the rev range.

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Lotus-Ford Twin Cam, 1558cc:
.................................... Europa .............. Elan ........... Elan
.................................... Twin Cam ......... Standard .... Sprint
Carb Type ................... DHLA 40 .......... DHLA 40 ..... DHLA 40
Choke ......................... 30 mm ............... 30 mm ........ 33 mm
Auxiliary Venturi .......................................................... 7848-1
Main Jet ...................... 115 ................... 115 ............. 120
Main Air Corrector ...... 160 ................... 210 ............. 130
Main Emulsion Tube ... 7772-1 .............. 7772-5 ........ 7772-5
Idle Jet ........................ 50 ...................... 50 ............... 50
Idle Jet Holder ............. 7850-2 .............. 7850-1 ........ 7850-2
Float ....................................................... 7298-1 ....... 7298-01
Float Weight ................ 8.5 gr ................ 10 gr .......... 10 gr
Float Setting Hgt, (mm) 14.5-15.0 ........... 14.5-15.0 ... 14.5-15.0
Float Needle Valve ...... 7180.15 ............. 8811-150 ... 7180.15
Pump Delivery ............. 8cc/ 20 strokes... 7.5cc/ 20 .... 8cc/ 20
Pump Jet ..................... 8083.40 ............. 7851.33 ...... 8083.45*
.................................. *if necessary to improve driving: 8043.35
Starter Jet ................... 70 ....................... 70 .............. 70
Starter Emulsion Tube, 7482-1 ............... 7482-2 ........ 7482-1
Fuel Pressure, (psi) ... 1.25 - 2.5 ........... 1.25 - 2.5 .... 1.25 - 2.5
Slow Running Speed ... 800 rpm ............. 800 rpm ....... 800 rpm
Air Trumpet Length ....................................................... 4.0cm/ 1.575"

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Lotus Service Bulletin
Class: ........ 111
Number: .... 1972/11
Type: ......... Elan Sprint & +2S 130
Date: ......... 09.06.1972
Title: .......... Dellorto Carburetters fitted to Elan 'Sprint' & +2S 130
Reason: .... To show the differences to Technical Data as published
................... in the Respective Workshop Manuals, and where appropriate
................... Owner's Handbooks.
Parts Req'd: See Separate Parts Bulletin.
Action: ........ The differences to Technical Data, which affect both
.................... models is as follows:

Carburetters (2):
Type: ..................... Dellorto 40 DHLA

Settings:
Choke: ................. 33 mm.
Main jet ............... 120
Idle jet .................. 50
Pump jet ............... 45
Starting jet ........... 70
Main air corrector, 130

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Lotus Service Bulletin
Class: ........ 111
Number: .... 1973/10
Type: ......... All Models
Date: ......... 02.04.1973
Title: .......... Dellorto Carburetters - Type DHLA 40 and DHLA-E 40.
Reason: ..... Inform Dealer personnel of recommended revised float level adjustment.
Action: ....... This information supersedes that already published
................... in Section V (Fuel System) of the Workshop Manuals.

Two float assemblies are currently used in Dellorto carburetters, these being:
Float Ass?y: 7298-01 (A036 E 6061Z) used in DHLA 40 (non-emission) carburetter.
Float Ass?y: 7298-02 (A036 E 6062Z) used in DHLA-E 40 (emission) carburetter.

Float Levels: 14.5 to 15 mm - Type DHLA. 40 (7298-01, 10 gram Float)
Float Levels: 16.5 to 17 mm - Type DHLA-E 40 (7298-02, 8.5 gram Float)

To check the float level, the whole assembly is held with the floats hanging down and needle valve lightly closed. To adjust the float level, the arms should be bent.

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Standard Generic (not Lotus) starting point (tune from here):
Tuned for ................... POWER .... FLEXIBILITY
Carb Type .................. DHLA 40 ... DHLA 40
Choke ........................ 31 mm ....... 29 mm
Main Jet ..................... 125 ............ 120
Main Air Corrector Jet, 170 ............ 170
Main Emulsion Tube ... 7772-7 / 7772-6 for both
Idle Jet ........................ 45 .............. 45
Idle Jet Holder ............. 7850-2 ...... 7850-2
.................................... (weaker .6 or.1, richer .
Pump Jet ..................... 35 / 40 ....... 35 / 40
Starter Emulsion Tube, 7482-1 ....... 7482-1
Auxiliary Venturis ....... 7848.3 ......... 7848.3

[William2]

Many thanks for your comprehensive reply. My DHLAs' are type G. I hope they are compatible with Elans.