View single post by Esprit2
 Posted: 11-02-2005 01:58 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 320
Brett Gibson JH5 20497 wrote: Tim, how do you know which spec Dell's you have, mine are 40'E's, on a stock engine, would it show on the Dell tags...It was recommended I use anti-spit back things on the I dont have pump jet filter/springs installed, do I need to get them ???


First I should clear up this "Spec" business.   Lotus used "Spec" numbers for the engines used in different years/markets/models. When I talk about a Spec-X Dellorto,  I'm referring to the Dellorto configuration used on the Lotus Spec-X 9XX engine.   Call EuroCarb and ask for the jets for a Spec-X Dellorto and they won't know what you're talking about.

Spec 1 1974 DOM 907,  Dellorto DHLA45E,  38mm choke (too big)

Spec 2 1974 USA 907,  Zenith Stromberg

Spec 3 1975 DOM 907,  Dellorto DHLA 45E,  35mm choke (too small)

Spec 4 1975 USA 907,  Zenith Stromberg

Spec 5 1976-80 Euro 907,  DHLA45E,  36mm choke (good)

Spec 6 1976-79 Calif / 1976-77 49-State USA,  Zenith Stromberg

Spec 7 1978-80 49 State USA 907,  Zenith Stromberg

Spec 8 1980 California, USA 907 ,  Zenith Stromberg

Spec 9 1980 onward,  Euro 912 Lo-Comp,  DHLA45E,  37mm choke

Spec 10 1986 onward,  Euro 912 Hi-Comp,  DHLA45D,  37mm choke

Nothing on the carb body itself tells you what Spec it is.   If the tags are still intact,  then a good Dellorto source like EuroCarb may be able to tell you what car/engine they are from and what the original jetting was.   If the carb happens to be from a Lotus,  then you could relate the jetting combination back to the Spec-number via the data Lotus published in it's manuals.

I don't have all the relevant tag numbers,  just a few for Lotus applications.   However,  If you have 40E's,  then they are NOT Lotus and hence,  are Spec-nothing (the Spec-X being Lotus-only terminology).   Lotus used DHLA40's only on the J-H 907's,  but didn't refer to them as a Spec-something.   All "Lotus" naturally aspirated 9XX engines got DHLA45's.   Some model-years of DOM/ROW carb-Turbo 910's got 40M's,  some got 45H's or 45M's. All Fed carb-Turbos got 45M's.

The best Lotus manual to refer to for a 9XX jetting history is the 1983-87 S3 & Turbo Esprit.   It includes the carb data for those years plus all years prior for "Lotus" carb-9XX engines (1974 onward Lotus,  but not J-H). Section TDA (Technical Data, A) covers the naturally aspirated 907/912 engines,  and TDB covers the carb-Turbo 910's.

BTW,  I think every naturally aspirated 9XX owner (Lotus or J-H) should have a copy of TDA (TDB optional) to supplement their Shop Manual Technical Data Section.   I've posted them in PDF format on my son's website:     (TDA.pdf  and  TDB.pdf)

33mm chokes are too small.   Stock J-H 35's are too small to start with (IMHO),  and with 33's it's like having both hands firmly gripping the poor thing's throat.

You wrote, "Anti-spit back things…"

??    Anti-reversion soft mounts, maybe?

Lotus' metal soft mount spacers have a tapered bore with a lip. Like a funnel pointing down stream (looks like something from Dr. Kamm's theories). At first glance it appears to be a flow restriction, but it actually helps. There are some really weird flow patterns taking place in an intake manifold including reverse flow/ reversion waves. The anti-reversion spacers block the reversion wave pressure pulses that would otherwise screw up the steady vacuum (depression) the carb needs to meter properly. Weber took a different approach on some of it's carbs (mostly downdraught). An extension tube from the venturi can be seen sticking out of the carb throat/ air horn. That's so the venturi can breathe reversion-free outside air. But I digress…

Anyway, anti-reversion soft mounts are a good thing if you can find them. They're also a Lotus-thing and you won't find them just anywhere. I've not seen the reversion lip on any aftermarket soft mount spacer.

You wrote,  "…plus I dont have pump jet filter/springs installed,  do I need to get them ???

:-/      ...Huh?

The springs on the accelerator pump linkage?   Absolutely you need them !   They're what operate the pump,  not the pushrod directly.   Without the springs the pumps won't work,  the engine will stumble/ fall on it's face whenever the throttle is opened,  and the car will be a bear to get off the line.

Or are you talking about something else?


7850.7 Idle Jet Holders (Idle Air Correctors) are about right.   I use them with a 55 Idle Jet as a starting point with naturally aspirated 9XX's.   The Holders are usually about right,  but the Jet sometimes needs tweeking to suit the individual engine.   52's or 53's might be big enough for a stockish Federal J-H 2.0 907.

To jet the idle circuit, start with the Holders.   Open the throttle gently. If the engine hesitates off-idle,  the Idle Air Corrector Jets (Idle Jet Holders) are too lean.   Install a step or two richer and try again.   When the Holders are set,  move on to the Idle Jets.

If the engine stumbles under full throttle somewhere in the 3200-4000 rpm range,  then the idle circuit is running out of capacity before the main circuit takes over.   Increase the Idle Jet size until the stumble just disappears,  no more.   If a large change in jet size is required,  then the change may also affect the air corrector.   Go back and repeat the full cycle of checking the air corrector,  then jet.

For the same air flow  (ie, feeding the same engine),  smaller chokes result in higher air velocities through the carbs.   Higher velocities mean a stronger vacuum in the venturi which can in turn,  pull the fuel over more quickly…  ie,  better throttle response.   With smaller chokes a slightly richer overall condition exists and it's usually possible to go with slightly smaller mains.   You went smaller chokes and larger mains.   I can't tell from where I'm sitting if your engine needed that,  but it's a combination I wouldn't normally have expected.

With 8.5g floats,  I usually run the float heights at 14.5-15mm.   Lighter floats need to be set higher to maintain the same fuel level. Light floats set lower (16.5-17.0mm) will cause an overall lean condition in the carb.

If I were going to play with your carbs,  I'd probably start by putting the missing springs on the accelerator pump (if that was what you were talking about) and set the pump stroke.   Then put the chokes and mains back where you found them (well,  okay,  if it were me,  I'd go with larger 36mm chokes… but lets not introduce that debate here).   Keep the  .7 Idle Jet Holder and go with a  53,  55  or 56L Idle Jet.   Then set the float height to 14.5-15.0mm to go with your 8.5 gram floats,  balance the carbs with a 4-tube manometer  and adjust the Idle Mixture Screws to give peak vacuum.

Balancing is key.   Good carbs will run like crap if the balance is off just a little bit.    And the balance will be off when you install the carbs…   almost guaranteed.

Then see how it runs and go from there.

Good luck,


Last edited on 11-02-2005 11:52 pm by Esprit2