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correct manifold for 40 mm Dellortos?  Rate Topic 
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 Posted: 06-25-2019 03:18 am
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sscheffel
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I have the manifold and carbs off the car for some head work.. looking thru the manifold tubes into the carbs there is a pronounced lip where the carbs and manifold join. The diameter of the manifold appears greater than that of the base of the carb. The manifold has a cast part number 907E 0123Y. Are the diameter of the manifold tubes all the same, or do I have a manifold for 45 mm Dells? I would assume the throats of the both the manifold and carbs would match up providing a smooth transition between these two items.. Any insight would be most appreciated.

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 Posted: 06-25-2019 04:42 am
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Esprit2
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You stated that the Dellortos are 45mm DHLAs, and you quoted a Lotus part number for the manifold. That would be a match. All Lotus 907, 911, 912LC and 912HC engines used 45mm DHLAs. Only the Jensen-Healey 907 used 40mm DHLAs, and I'm not aware of the JH intake manifold is uniquely JH/ 40mm, or just the same Lotus manifold. Off topic, but perhaps the owner of a UK/ Euro/ ROW JH with an original Dellorto intake manifold can look for a similar part number on it.

Three 'Lotus' Dellorto intake manifolds were used through the years, and they're all basically the same... they all have the same bore diameter. They just moved heater hose spigots around and grew additional drilled & tapped holes for later thermal vacuum switches (emissions stuff, easily plugged), and that warranted different part numbers. The induction portions & mounting flanges were all the same.

The gating item as far as bore diameter is concerned would be the bespoke Lotus die cast spacer plate for the soft mount. It's bore is funnel shaped. The bore is larger at the carb flange, and smaller at the manifold flange, with a lip extension going slightly into the bore. The small end is smaller than the manifold bore. That may sound restrictive, but it actually improves flow and carb performance. There are all sorts of funky flow dynamics going on in an intake manifold, including pressure pulses, reversion waves and reverse flow. That funnel/ lip was developed via Fluid Dynamics (KAMM theory) in order to scrape any reverse flow off the wall, cleaning up flow, and preventing them from getting inside the carb and screwing up the metering dynamics. If you have the Lotus spacer plates, keep them. They're the best available, the only ones with that KAMM lip feature, and none of the aftermarket plastic spacer plates, or the thin steel plates with over-molded O-rings work as well.

JH p/n 93552 = J-H Manifold, used with 40mm Dellortos. I don't know if it's the same as...

B907E0123Y = Lotus Manifold, 907 & 912LC.
A912E2184K = Lotus Manifold, 912HC, more emissions fittings.
B912E2184K = Lotus Manifold, 912HC, even more emissions fittings.

B907E0129Z = Lotus Spacer/ Soft Mount, used with all 907, 911, 912 engines.
A907E6027Z = O-ring, Soft Mount, used with all.

Regards,
Tim Engel

Last edited on 06-25-2019 04:44 am by Esprit2

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 Posted: 06-25-2019 12:38 pm
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sscheffel
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Tim

thanks for your reply. I was hoping that you would see my post and provide your insight.. Much appreciated.. point of clarification.. I have 40mm Dells on the car.

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 Posted: 06-25-2019 02:54 pm
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Esprit2
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sscheffel wrote:
or do I have a manifold for 45 mm Dells?Sorry, I read faster than I comprehended. I was busy writing my reply, glaced at your message to see what size Dellortos you have, and just saw "45 Dells".

Which gets us back to... I don't know the specifics of the manifold used on the J-H 907. Was it just the Lotus manifold (still two years away when the J-H came out), or was it a bespoke J-H manifold for 40 Dellortos? Perhaps a UK/ Euro/ ROW owner of a J-H with Dellortos can report a part number that is cast into the J-H manifold, and hopefully measure the bore.

A manifold with any Lotus part number only came in one size, and Lotus used them with 45mm Dellortos.

Is your original question just a point of curiosity, or is the engine running poorly in a way that makes you suspect the manifold might be a contributor to the problem?

Tim

Last edited on 06-25-2019 03:59 pm by Esprit2

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 Posted: 06-25-2019 04:42 pm
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sscheffel
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The prior owner installed the dells. He thought they may have been off an Alfa. according to him, the car had some cam work 104 and 107, higher compression pistons on a rebuild of 8,000 miles ago. The car runs OK, but I always thought there should be more grunt.also a common theme on a hot engine is popping upon letting up on the accelerator, regardless of jetting.. Never happens when engine is cold. I saw your very comprehensive postings about jetting and all the recommendations which was insightful.. I played lot with jetting but no real improvement on popping and power.
Upon looking further I discovered my 40s have size 30 chokes. so maybe that is part of the problem.. The JHPS site set up I believe calls for 34s which I have on order. My mechanic is suggesting buying some 45mm Dells especially after seeing this less than smooth bore transition between the carb and manifold. there is a site in Italy that takes 40s and bores them out to 45s.. Any thoughts on that option? I live at an elevation of 5200 if that makes a difference.

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 Posted: 06-26-2019 04:22 am
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Esprit2
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Alfa carbs, eh? Is there any suffix letter after the throat size... like 40C or 40E? When you get to 40H & beyond, those are 'emissions' carbs that sorta re-invented how DHLAs operate. They're a whole 'nuther discussion.

30mm Chokes ?! Those 'are' small for a 2.0 or 2.2. Even the Elan/ Europa's small 1.5 litre Lotus Twin Cam used Dellortos with 30-33mm chokes. I'm not aware of what chokes the JHPS site calls for (I'll take your word that it's 34), but the J-H factory DHLA 40s used 35 chokes. However, the standard jetting won't be of much good to you at 5200'... altitude is a very big deal to carburetors. The air gets thinner so the mixture gets richer. At 5200', a bone stock Dellorto J-H should be so rich that it will bog down, lose power, and belch black smoke. It needs to be leaned out. Despite the hiccups you mentioned, your engine reportedly runs well enough to indicate the carbs have already been tuned leaner than J-H stock.

Unfortunately, I have no experience with carbs at high altitudes... I'm a flat lander at 900'. However, Lotus Owners of Colorado (LOCO) has members running 907 engines in the Denver area, and J-H owner Jeff Rehman has recently completed a 2.2 conversion on his 907. There are folks who should be able to help you out. Jeff is using Weber DCOEs, but he worked through the altitude issues. Perhaps he could give you some pointers. I believe he worked with LOCO member Vic Cooper, who not only maintains his own very nice hotrod Lotus Esprit, but is an automotove pro who understands tuning for altitude. If you don't know Jeff, PM your contact info to me, and I'll forward it to him. Or go to the Jensen-Healey Facebook page... he hangs out there.

As for the popping on over-run. Have you played with the Idle Jet Holders (ie, Idle Air Correctors), or just the Idle Jets? Is there any off-idle hesitation?

Boring 40s out to 45s ? I've not heard of anyone doing that... no success or failure reports. Personally, I'd be more comfortable buying the size I wanted in the first place.

How are your Alfa Dellortos jetted? How about adding a column to the following:

Stock is TOO RICH at 5200 feet.
Jensen-Healey, Stock 2.0L, 8.4:1, C-cams . . . . . . . Your Alfa Dellortos
Carb Type. . . . . . . . . . . . . . . . . DHLA . . . . . . . . . DHLA
Throat, Emissions . . . . . (mm). . 40E. . . . . . . . . . . 40__ (suffix ??)
Throat, NON-Emissions . (mm). . 40. . . . . . . . . . . .
Choke . . . . . . . . . . . . . (mm). . 35. . . . . . . . . . . . 30, 34 on order

Main Jet . . . . . . . . . . . . . . . . . . 130 . . . . . . . . . .
Main Air Corrector Jet. . . . . . . . . 160 . . . . . . . . . .
Main Emulsion Tube . . . . . . . . . . 7772-5. . . . . . . .

Idle Jet, Emissions. . 5-Speed . . . 52L . . . . . . . . . .
Idle Jet, Emissions. . 4-Speed . . . 55L . . . . . . . . . .
Idle Jet Holder, Emissions . . . . . . 7850-5, LEAN . . .

Idle Jet, . . . . . NON-Emissions . . 50. . . . . . . . . . .
Idle Jet Holder. NON-Emissions . . 7850-1, Normal. .

Float Weight . . (grams) . . . . . . . 8.5 . . . . . . . . . .
Float Height . . (mm) . . . . . . . . . 16.5-17.0 . . . . .
Pump Jet. . . . . . . . . . . . . . . . . . 45. . . . . . . . . . .
Starter Jet. . . . . . . . . . . . . . . . . 70. . . . . . . . . . .
Starter Emulsion Tube. . . . . . . . . 7482-1 . . . . . . .
Needle Valve . . . . . . . . . . . . . . . 7180-15 (170) . .
Slow Running Rpm, 4-Spd . . . . . . 800- 850. . . . . .
Slow Running Rpm, 5-Spd . . . . . . 950-1000 . . . . .
I Prefer a Little Faster. . . . . . . . . . 1000-1100 . . . .

Idle Air Corrector Jet Sizes are in Random Order:
Leaner . . . . . . . . . . . . . . . . . . Normal . . . . Very. . . . . . Richer
7850-5, . . -10, . . -9, . . -4, . . -1, . . -3, . . -6, . . -7, . . -2, . . -8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Similar

Regards,
Tim Engel

Last edited on 06-27-2019 04:22 pm by Esprit2

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