Lotus Elan

Weber DCOE 40 type 31

PostPost by: Concrete-crusher » Mon Jun 05, 2017 12:35 pm

Should the weber dcoe 40 type 31 carbs have a small hole in the throttle plates ?

thanks Steve
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PostPost by: el-saturn » Mon Jun 05, 2017 2:07 pm

mine do steve! sandy 36 / 4982
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PostPost by: fatboyoz » Mon Jun 05, 2017 6:39 pm

Mine too Steve.

Concrete-crusher wrote:Should the weber dcoe 40 type 31 carbs have a small hole in the throttle plates ?

thanks Steve
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PostPost by: RichardHawkins » Mon Jun 05, 2017 7:14 pm


My carburettors also have a small in the throttle plates, 1mm if I remember correctly. I thought the purpose was to balance the air flow at tickover, but in that case I would have expected the holes to be of different sizes, but they are not. I have been told that the hole is to let sufficient air into the engine whilst still keeping the throttle plate in the correct position with respect to the first progression hole. This seems reasonable, but I don't have personal knowledge if this is correct, and why don't all Webbers have to his hole?

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PostPost by: Craven » Mon Jun 05, 2017 7:44 pm

I put this on the forum a few years back.
Check you have the correct throttle plates in you 31?s, the small hole was added in an attempt to retard the progression, a basic problem with 31?s on a Twincam. Apparently it improved things when engines were new but later the fundamental fault returned giving the lumpy over rich tick over.
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PostPost by: NickD » Mon Jun 05, 2017 8:54 pm

This subject was covered in some detail in John Passini's excellent "Weber Carburettors" book, which is, in my opinion, the best written and most readable book on Webers available. It was originally written in the 60s and contains several references to the Elan. A rare-ish book now, but can still be found - eg:

https://www.abebooks.co.uk/978094798167 ... 981675/plp

The first progression hole on the type 31s was apparently drilled too far forward and so was uncovered at too small a throttle opening. Throttle plates with holes were fitted to offset this effect as a smaller initial opening at idle was then required. This could cause a problem if/once the carburettors became worn and leaky as they could then allow too much air through to allow a slow idle.
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PostPost by: prezoom » Tue Jun 06, 2017 2:40 am

A plus 1 on Passini's Tuning book for Webers.
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PostPost by: Elan45 » Tue Jun 06, 2017 3:03 am

But how do DCOE carbs get worn? The throttle shafts are mounted in tiny ball bearings. I suppose the bearings could wear, but the action would then be so gritty, that no one would want to use them.

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PostPost by: Chancer » Tue Jun 06, 2017 6:45 am

The spindle seals, early carbs used o?l soaked leather and a spring.
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PostPost by: rgh0 » Tue Jun 06, 2017 1:54 pm

There are multiple variables in Weber carbs fitted to Elans that potentially affect whether a hole in needed in the throttle plates or not and how many of those variables are ex factory or after market are not clear in typical Lotus fashion.

I am not sure if any Webers originally came with the holes from the factory but I know lots have had holes drilled subsequently. My 1968 S4 had DCOE40 - 31 that did not have holes and did not need them. Those carbs are now on my PLus 2 and also does not need holes in that engine. But I know of many people who have drilled holes to get the idle speed right and to also eliminate a lazy return to idle or a stumble off idle. Most of this has been required on early DCOE-18's in my experience rather than on the later DCOE-31

The 18's versus the 31's had a different progression hole pattern and spacing, There are also at least 2 different throttle plate types with different closing angles and finally there is all the differences in what an individual engine needs for air flow at idle due to jetting, cams, compression, friction and leakage versus what the carbs supply without affecting progression hole exposure which has to happen just right to get a smooth off idle acceleration and a proper return to idle.

I have never explored all these possible combinations and permutations but have just solved what was need to get my individual engines to run right. There is no simple magic answer that is properly documented but the Weber books mentioned give at least a general guide. Keith's sidedraft_central yahoo group is probably a more useful resource if you can spend the time working through all the relatively unstructured information there around this topic.

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PostPost by: Concrete-crusher » Tue Jun 06, 2017 5:55 pm

Thank you all for the replies , it's helpful to understand the reasons for the holes. Being old carbs , they are probably now letting too much air though despite a rebuild.

My main problem is a rough idle below 1100 , above that the engine is running fine and the plug colours look OK as a light brown.

I'm wondering if the situation might improve by putting in a higher octain fuel 98 instead of the 95 that's in there at the moment.

Also I did,not think the plates were giving a particularly tight seal , can the plates wear or is it likely they are just not centred ?

I plan to spend a bit more time on them this weekend , getting there slowly

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PostPost by: mbell » Tue Jun 06, 2017 6:41 pm

It's quite possible there too much of gap around the plates and that with the hole provides too much air at idle.

I spent quite a bit of time on my DCOE 31 trying to close the gap and balance between the barrells but never got anywhere near happy with it and in the end i just bought a new set of 151's as didn't want to throw more money at the old carbs.

Where do the plates sit relative to the first progression hole at idle? You should just be able to see the edge of the plate through the hole. If you can't see it or some on the hole on the engine side of the plate then there is likely an issue there.
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PostPost by: Craven » Wed Jun 07, 2017 10:08 pm

Only real problematic wear problems I?ve seen is where worn bearings have allowed the throttle plate to contact the barrel neatly machining away the bore. Dellorto or Weber, in perfect condition with correctly centred throttle plate there would be no light visible ( when closed ) around the edges or at the spindle if viewed as if a telescope.
In operation the throttle plate is never fully closed, poor idle is probably due to over rich mixture IMHO an inherent problem with 31?s.
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PostPost by: nmauduit » Thu Jun 08, 2017 8:15 am

Concrete-crusher wrote:
My main problem is a rough idle below 1100 , above that the engine is running fine and the plug colours look OK as a light brown.


Hello Steve,

did you check the balance of idle air flow with a flow meter (using a synchronizer, the snail type is cheap and convenient though does only one cylinder at a time) ? unless you have similar quantities of air getting into each cylinder at idle, which relies on butterflies being well centered then carbs linkage adjusted, it is not possible to get smooth idle.

Only after the flow is balanced the best I can (one needs to remove the carbs to access the butterflys if intra carb imbalance is too high) I spend time tuning idle by the spring screws. When you take off from idle, air starts flowing around the butterflies so does not depend so much on centering, which removes the unevenness that may be at idle.
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PostPost by: matt1954 » Fri Jun 19, 2020 3:20 pm

Can anyone help, I’m putting my 40 DCOE carbs back together but can’t remember whether the small hole in the throttle plate should be at the top or bottom. I’m guessing top but I could be wrong.
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