Lotus Elan

Webber carburetors

PostPost by: Bowers » Tue Sep 05, 2023 7:34 pm

Hi all I have a lotus elan plus two.
When looking at my carbs I can see they are missmatched
there is one 40dcoe type 18 and one 40 dcoe type 31
Is this a problem ?

I do have a set of
40 dcoe 151 s
However these are sorely needing overhauled.
Do you guys recommend any companies for this ?
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PostPost by: rgh0 » Wed Sep 06, 2023 8:03 am

It may just be that a early DCOE 18 top cover has been fitted at some stage and the actual carbs are identical. The carbs are basically the same except for the progression hole drilling I believe so check if the progression holes are the same.

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PostPost by: Bowers » Wed Sep 06, 2023 10:00 am

HI Rohan

How do I check that. Sorry total newbie .
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PostPost by: SENC » Wed Sep 06, 2023 12:14 pm

Back near the idle mixture screws you'll see caps screwed into wells. By memory, these are between 5mm and 10mm in size. Unscrew those caps and peer down into the wells (with a light) and you'll see the progressions holes. Note the number, size, and orientation as these changed across types. If the air intake side of the carbs are open (no airbox or filters), you might find it easiest to grab photos for comparison by sticking a flashlight/torch in the intake and photographing down the progression hole wells. Take them with the butterflies/throttle both closed and open.
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PostPost by: SENC » Wed Sep 06, 2023 12:18 pm

IMG_20230906_081723.jpg and
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PostPost by: Bowers » Wed Sep 06, 2023 12:48 pm

If the progression holes are the same does that mean they ate the same type ?
If that's the case how will I know what ones what. They both have 3 holes does that mean they are a type 31 or 18

Thanks for the help
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PostPost by: elans3 » Wed Sep 06, 2023 12:52 pm

Accelerator pump stroke is also different.on 31's. It's 10mm on 18's, can't quite remember what it is on 31's, either 12 or 15mm I think. However, as above, most likely that a top has been exchanged at some time during its life.
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PostPost by: SENC » Wed Sep 06, 2023 2:42 pm

Bowers wrote:If the progression holes are the same does that mean they ate the same type ?
If that's the case how will I know what ones what. They both have 3 holes does that mean they are a type 31 or 18

Thanks for the help

I'd bet there is a resource out there on the web that details the progression holes by type, but I didn't turn one up in a quick Google search.

If you join the io group at https://vintagetechnologygarage.groups.io/g/sidedraft you'll find some photos of progression groups in threads and also in the photos section. Might be useful in identifying yours. Note, though, that just because both have 3 progression holes doesn't mean they are the same as progression hole diameter and location can differ (significantly or slightly) between types.
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PostPost by: Craven » Wed Sep 06, 2023 3:25 pm

Unless you remove the cab’s and can make some fairly accurate sub mm measurements you are very unlikely to identify which is which by eyeballing the progression holes. A better way will be, ensure the carb throttle plates are fully closed in both cases, with a 31 the 1st that is the largest of the three holes will be completely blocked off by the brass edge of the throttle plate when viewed from above, do the same test on the other carb. On an 18 the 1st hole will be only part blocked, hence my argument that the 31 progression is way too far advanced for a 1558cc Twincam.
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PostPost by: baileyman » Thu Sep 07, 2023 11:13 am

Bill McCurdy had a way with progression holes to get over the dead spot. He explained something about boosting the richness at just the right place, perhaps even adding a hole, but I'm not sure, and he can no longer provide details. The 45s I have just go and go at any throttle position.

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PostPost by: ericbushby » Thu Sep 07, 2023 11:58 am

Hi All,
Apparently there is weakness in the positioning of the progression holes on the Spanish made Weber 151 carburettors, and maybe also on the 152. The position of the first hole is such that as the butterfly moves off it`s idle position it lets air pass before exposing the first petrol hole. This causes a slight weak mixture moment, sometimes called an `off idle stumble`.
After following the advice of Keith Franck and reading a very detailed description of this problem which I found on the internet I decided to modify it.
Keith sells a jig for drilling another 1mm progression hole 1mm downstream of the existing first hole to cure this.
Without a jig it took a lot of measuring and drawing until I finished up with a tiny centre pop in the required position.
Being a scaredy cat, I went for a 0.8mm hole.
This cured the stumble immediately.
It took a lot of courage, measuring, and thinking spread over two days to gather the courage to do it and about one minute for each hole.
Hope this is helpful
Eric in Burnley
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PostPost by: 2cams70 » Thu Sep 07, 2023 1:33 pm

elans3 wrote:Accelerator pump stroke is also different.on 31's. It's 10mm on 18's, can't quite remember what it is on 31's, either 12 or 15mm I think. However, as above, most likely that a top has been exchanged at some time during its life.


This I believe is a good easy way to distinguish a 31 from an 18 assuming these parts are original. I checked the specs. and an 18 has 10mm stroke pump rod and the 31 a 16mm stroke pump rod. If you have a cover that says 18 and 10mm stroke pump there’s almost a certainty that it’s an 18.

You do need matched carbs as a starting point
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PostPost by: types26/36 » Thu Sep 07, 2023 2:09 pm

I could be wrong but I thought type 31’s had a small hole in the throttle plate (butterfly) and the type 18’s did not....if true it would be a quick way to I.D. the type of carb, a top could be changed but its a bit more involved the change the butterflies. Happy to be corrected though:?:
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PostPost by: Craven » Thu Sep 07, 2023 4:31 pm

types26/36 wrote:I could be wrong but I thought type 31’s had a small hole in the throttle plate (butterfly) and the type 18’s did not....if true it would be a quick way to I.D. the type of carb, a top could be changed but its a bit more involved the change the butterflies. Happy to be corrected though:?:

Yes 31’s fitted to the Lotus Twincams, good point, did have an extra air bleed hole in throttle plate but not found on all 31’s. Not sure if it’s visible on fitted carb’s.
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PostPost by: elans3 » Fri Sep 08, 2023 2:21 pm

No hole in the throttle plate of my original 31's on two of my 3 S3's I've owned. The third car was an early S3 (non airflow) that had 18's.
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