Weber DCOE 40 18 or 31?

[Niklas777]

Hi all,

I am planning to buy a new set of webers for my lotus Elan S3 SE (36 7444) soon.
I was looking for the answer on the forum but could not find the response.

For this particular year/model, what is the OEM configuration?
Type 18 or 31?

Does it change anything at all?

Thanks in advance,
Niklas

[types26/36]

I used to have a 1967 S3 SE (36 /75XX) it was fitted with DCOE 18’s, from the parts manual it looks like 31’s were fitted from 1968 but don’t believe new 18’s or 31’s are available anymore (unless you find some NOS but unlikely.
The modern equivalent would be DCOE 151’s, do a search on the forum for 151 carbs as they will need jetting and I think there is should be some info what others have used.

[Niklas777]

I used to have a 1967 S3 SE (36 /75XX) it was fitted with DCOE 18’s, from the parts manual it looks like 31’s were fitted from 1968 but don’t believe new 18’s or 31’s are available anymore (unless you find some NOS but unlikely.
The modern equivalent would be DCOE 151’s, do a search on the forum for 151 carbs as they will need jetting and I think there is should be some info what others have used.

THanks a lot Brian,

By the look of this document the 68 S/E were fitted with DCOE 31's?

I found new DCOEs 18 so i was going for this, but if 31 were fitted then maybe 31.
Otherwise i've read a bit on 151's that seem to have some advantages (easier to sync) over the 18/31 but do require some advanced settings when fitted on the car, which is apparently not the case for 18's or 31's if i understood well?

[types26/36]

Yes Niklas, the 151’s have a idle by-pass system that enables you to individually adjust each choke (4) for air flow whereas the 18/31’s can only be adjusted in pairs (2) unless you start bending the throttle shafts.

[nmauduit]

Type 18 or 31?

Does it change anything at all?

I understand that the main difference that is not readily tunable would be the number/size/location of transition holes - unless one fancies redrilling transitions holes, there are some setups to do so, yet in this case I don't think that could not be compensated by a suitably careful jetting and tuning...

[elans3]

A '68 Elan S3 would have had 31's.
The main advantages of 151's over the earlier carbs, is the aforementioned additional airscrews that make tuning and synchronisation much easier, but also that the 151's have steel throttle shafts running in small roller bearings with seals either side of the bearings, (so no old-fashioned leather seals as in the early stuff)..
This means that the shafts don't wear the bearings, thus vastly reducing the chance of air being sucked into the carb through the ends of the shafts, and they don't bend as easily as the brass shafts used to. I've been playing with DCOE's for 45 years+, and have never seen a set of 151's with throttle shaft play.
The ideal situation would be to fit 151's whilst you have the car, and provide a set of 31's if you ever came to sell it.
From 2009 to 2020 (before I retired), I had my own little business buying and selling Historic Motorsport parts, and in 10 years, I probably had 6/700 sets of DCOE's through my hands for refurb & re-sale.
I sold many a set to Elan owners who were delighted with the results, and I advised them all the same, keep the 18's or 31's for originality, but use the 151's whilst you have the car.
In my experience (also having fitted 151's to a couple of my previous Elan'sand my Twincam Alfas), most of the original jetting can just be transferred over, but 151's do tend to like the idle jets slightly richer. As each engine is different, you'd need to play with the idles to get a near perfect setting,
Hope that helps.

[Niklas777]

A '68 Elan S3 would have had 31's.
The main advantages of 151's over the earlier carbs, is the aforementioned additional airscrews that make tuning and synchronisation much easier, but also that the 151's have steel throttle shafts running in small roller bearings with seals either side of the bearings, (so no old-fashioned leather seals as in the early stuff)..
This means that the shafts don't wear the bearings, thus vastly reducing the chance of air being sucked into the carb through the ends of the shafts, and they don't bend as easily as the brass shafts used to. I've been playing with DCOE's for 45 years+, and have never seen a set of 151's with throttle shaft play.
The ideal situation would be to fit 151's whilst you have the car, and provide a set of 31's if you ever came to sell it.
From 2009 to 2020 (before I retired), I had my own little business buying and selling Historic Motorsport parts, and in 10 years, I probably had 6/700 sets of DCOE's through my hands for refurb & re-sale.
I sold many a set to Elan owners who were delighted with the results, and I advised them all the same, keep the 18's or 31's for originality, but use the 151's whilst you have the car.
In my experience (also having fitted 151's to a couple of my previous Elan'sand my Twincam Alfas), most of the original jetting can just be transferred over, but 151's do tend to like the idle jets slightly richer. As each engine is different, you'd need to play with the idles to get a near perfect setting,
Hope that helps.

Thanks a million for your answer. This forum's expertise is really appreciable for someone discovering the Lotus world.

Can't wait to finally enjoy my Elan

[Niklas777]

Yes Niklas, the 151’s have a idle by-pass system that enables you to individually adjust each choke (4) for air flow whereas the 18/31’s can only be adjusted in pairs (2) unless you start bending the throttle shafts.

Thanks Brian, very clear.

[Niklas777]

Hi all,

Just a quick update here.
I received and installed the Weber 151, applied the settings given by Lotus as OEM settings for a 68 S3 S/E
The result is not that great, with some flat spots at low rpms, uncomfortable progression and lack of power.
I cannot really count on jets that were installed on previous carbs as those carbs were not the original ones at all (it makes part of the discoveries you make when you buy this sort of car from an unreliable seller )
I have no choice but to start from scratch in order to get a decent result.

By looking at the information on the forum (and especially the ones given by @ElanS3), the factory settings for the S3 S/E were not really accurate and a fix was then added to the factory workshop, giving the following settings :

-Chokes 30mm X4
-Main jets 115x4
-Air correctors jets 200x4
-Emulsion tubes F11x4
-idle jets 50F8 x4
-Accelerator pump jets 40x4
-Needle valve 175x2
-Back bleed jet x2 around 50

Please let me know if those were the ones you applied on your S/E engines!

Whereas the prior factory setting were giving 32mm chokes and 150 air correctors jets, this might be the reason why yesterday's test was a failure...

I will therefore try with the updated settings after the summer holidays and let you know.

Niklas

[EPA]

I have 151 Webber’s and I find that 50F9 idle jets made a substantial improvement
Ed

[mbell]

Have you checked/measured the fuel level?

The fuel level has significant effect on operation of the carbs, especially transition, so it's wise to get it correctly set as starting point.

I think the factory measurement is somewhere around 27/28mm down from top of jet holder. Kieth franks recomends 25mm. I've found that to be a bit high for my car and can never tune out rich transition. So currently using 26mm.

[Niklas777]

Have you checked/measured the fuel level?

The fuel level has significant effect on operation of the carbs, especially transition, so it's wise to get it correctly set as starting point.

I think the factory measurement is somewhere around 27/28mm down from top of jet holder. Kieth franks recomends 25mm. I've found that to be a bit high for my car and can never tune out rich transition. So currently using 26mm.

Thanks, would this also be applicable to a 68 s3 s/e?

[mbell]

Yes.