Lotus Elan

WEBER 45 DCOE 15/16

PostPost by: Stig » Wed Nov 17, 2021 4:36 pm

I am relatively new to the Lotus world.
My S4 SE does not run quite well. It idles OK and at over around 3000 RPM it is also OK but not in between when it spits and misbehaves. I have Weber 45 DCOE 15/16 carburettors. I understand that the Elan originally came with 40 DCOEs. Are the 45s suitable and will it be possible to tune them properly (by someone who knows his stuff)?
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PostPost by: Evante » Thu Nov 18, 2021 3:28 am

Hi Stig,

I would suggest you take a look at this paper by redline that goes into a clear description of the 5 circuit of the weber carb. From there you will be able to sort out where your problem is coming from. At least your will be able to have a background for the information others will offer.

https://240260280.com/Tech/Carbs/Weber/DCOE%20Theory%20Operation%20and%20Tuning.html

Also, in order to answer your question, more information will be needed about your engine and sizes of the jets, emulsion tubes, chokes, etc. in your carb. Just knowing that it is a 45 vs 40 is not really enough. For example, is your engine stock or modified? If modified, what is the cc, type of cams, valve sizes, timing?

Further, does you problem come in when you are under steady throttle, hard acceleration, slow acceleration?

I think the paper will give you some good background to understand why these issues are important, otherwise, the Weber Carb will seem like a mysterious black box.

Best,

JAS
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PostPost by: SENC » Thu Nov 18, 2021 3:38 am

I'm still learning Webers, so can't offer a lot of advice, but in addition to this site I'd also recommend registering and posting your question here:

https://vintagetechnologygarage.groups. ... aft/topics

Lots of Weber experience there, too, and you'll get plenty of advice.

Questions you'll be asked... what chokes do you have in the carburetors? What idle and main jets?

It sounds like you're describing the off-idle stumble these carbs are often known for, as I understand it is generally due to a over-rich idle jet (needed for smooth idle, but fat in the mid-range) - but again, I don't have enough experience with these carbs to really diagnose or advise.
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PostPost by: stugilmour » Thu Nov 18, 2021 6:04 am

Welcome to Lotus Stig.

Agree completely with the above comments. Get the basic information on your current setup and post on the Sidedraft forum in the above link. Prior to posting, suggest going through this “White Paper” that Keith Franck wrote:


An Alternative Way of Extracting the Best Tractability and Performance from the Weber DCOE Carburetor
© 2006 Keith D. Franck


https://www.rx7club.com/attachments/1st ... _paper.pdf

Note the White Paper was written a while ago, and he has greatly improved his products and understanding since. The basic idea is he improves the ability of the emulsion tube to lift fuel at part throttle, which allows the main jet to be sized for higher rpm and throttle opening without leaning out in the transition or running an overly rich idle jet to ‘hide’ the lean spot. He then uses the air corrector size to tune in when the main jet ‘comes on’; larger A/C and the mains start lifting fuel earlier to banish the flat spot.

Here is a short list of the basic info they will want to know to get you started

Idle jet size
Main jet size
Air Corrector size
Emulsion tube
Choke size installed
Fuel level (Keith recommends 25 mm, which is slightly higher than stock. See measurement details in his paper)
Accelerator pump spec you have it.

The particulars of your Twin Cam best you know, but I would not get too hung up on the details. Basically is it over-bored or stroked? Which cam is installed? If you don’t know I don’t think it will be a big deal as Keith is so familiar with various Twin Cams he can probably get pretty close.

I am also new to tuning Webers, and got great help from Keith on his forum. He makes custom idle jets and emulsion tubes that can really help with exactly the issue you seem to be having, which is commonly caused by going lean in the transition from the idle/progression circuit to the main circuit. He will send you his stuff by post, and can probably make a few recommendations right away to get you started.

I am not familiar with your exact model 45’s, but you may be able to tune them for your TC. It really depends on the details of the progression holes that your carbs have. It will be critical to know what choke size is installed though. The guys on the forum will likely have experience with your 45’s and be able to advise how you can effectively tune around the existing progression.

Another key point before trying to tune out the dreaded flat spot. Ensure your fuel supply is in good shape and delivering steady fuel pressure, the timing and ignition is spot on, and you have a wideband O2 gauge installed to read air fuel ratio (AFR) while making test runs.

Note even with Keith’s VP tubes you will need a selection of main jets and air correctors to test and optimize. You can get a good selection in kit form, and fill in as required from eBay.

For Keith’s idle jets, each jet assembly has air corrector holes that can be selectively closed off, so you hopefully only need one size available to tune your circuit. Note you can remove the main jets & emulsion tubes completely and test drive on just the idle circuit to confirm your idle jet size is working OK to around 2500 to 2800 rpm at part throttle, at which point it will crap out. You can then order the same size W jet from Keith if you are unable to fully tune out the transition with his VP tubes.

Hope this helps. Apologies for going on if you have tons of Weber experience with other marques. Basically the key message is your non-stock 45’s may be tuneable for your TC, but it is best to get a clear starting point and for sure know the installed choke size so you know what you are tuning to. I had never imagined tuning a carb a year ago, but I am now very happy with my newer 45-151’s, although my engine is decidedly non-stock and larger displacement.

HTH
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PostPost by: nmauduit » Thu Nov 18, 2021 11:50 am

Stig wrote:I am relatively new to the Lotus world.
My S4 SE does not run quite well. It idles OK and at over around 3000 RPM it is also OK but not in between when it spits and misbehaves. I have Weber 45 DCOE 15/16 carburettors. I understand that the Elan originally came with 40 DCOEs. Are the 45s suitable and will it be possible to tune them properly (by someone who knows his stuff)?
Stig


Welcome to the forum

a mentionned above, a key to spend your effort efficiently is to know where you stand, esp. the state of tune of the engine (standard road engine vs. full out race engine... including capacity if non 1600) and the current specs of the carbs (it is possible to have 45 covers on 40 carbs, the choke being a key factor in tuning the carbs). Last, if you have a lambda (richness) sensor on the car (or even access to one, though removable setups at the end of the exhaust are more prone to noisy data), the process will be greatly facilitated imho.
Last edited by nmauduit on Thu Nov 18, 2021 8:04 pm, edited 1 time in total.
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PostPost by: 2cams70 » Thu Nov 18, 2021 1:20 pm

Don't waste your time fiddling and wasting money purchasing new jets and chokes based on pot luck old wives tales and here say that may or may not improve the situation.

Spend your money wisely and have the carburettors properly set up by someone who knows what they are doing on a rolling road dynamometer.

Overly rich mixtures will cause excessive cylinder wear. Lean mixtures aren't good for your engine either. It's not just a driveability issue but the longevity of your engine at risk.
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PostPost by: Evante » Thu Nov 18, 2021 5:38 pm

I agree with 2cams70 that you should find an expert on webers and have him set you up properly. The guides that I and others suggested will be useful to get you oriented to the issues involved. However, actually doing the tuning yourself at the novice stage will be overwhelming and fraught with trial error, trial, error, and frustration.

It is best to go to an expert who will also take the time to teach you and show you what he does.

Best of luck and welcome to the forum.

JAS
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PostPost by: oldchieft » Thu Nov 18, 2021 10:31 pm

DCOE can be choked up to a maximum size or down to suite smaller engine

DCOE 40 chokes can be sized from 24mm to 36mm in 1mm increments.

DCOE 45 chokes can be sized from 28 mm to 40mm in mostly 1mm increments.
(the is no29 or 31mm)

DCOE 48 chokes can be sized from 40mm 41mm and 42mm

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PostPost by: SENC » Fri Nov 19, 2021 10:57 pm

Though I agree completely with 2cams and Evante that the easiest and best (and maybe cheapest in the long run) solution is a dyno and an expert, these are not horribly complex devices. I enjoy working on mechanical devices, and rebuilding mine and now learning to tune and adjust them has been a great way for me to learn. While one does need to take what is found on the web with an appropriate grain of salt, there is also a lot of great knowledge and experience - and, at least in my part of the world, truly knowledgeable folks are becoming harder to find. So, if you have that interest and proclivity, don't be scared off doing it yourself - it isn't a dark art.

The most obvious downside is the need to buy jets and other bits for setup and testing. I have a small kit set up - and once I get my Seven where I want it I would have no objection to sharing what I'm not using. Maybe we could get a few geographically proximate folks to pool bits that could be shipped back and forth for those in trial and testing mode.
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PostPost by: Stig » Sun Nov 21, 2021 4:10 pm

Thank you all for your feedback and advice.
I have another question. I have one Tipo 45 DCOE 15 and one 16 which I assume are a pair. There are also several other Tipos that seem to be used 9, 18, 31, 151, 152, etc. What are the difference and which ones are the most suitable and tunable?
Regards
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PostPost by: ericbushby » Sun Nov 21, 2021 4:32 pm

Hi Stig,
The 151 is a later version of the 40 DCOE and the 152 is a later version of the 45 DCOE
These have the air bypass screw on the side which makes balancing the air flow rate much easier than the earlier ones.
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PostPost by: SENC » Sun Nov 21, 2021 5:07 pm

Someone with specific knowledge of those types (tipos) will chime in, but different types often have different progression holes (different number, location, and/or size). You would generally want two of the same type, but perhaps the 45-15/16 are meant to be paired? Just remove the plugs and look at the progression holes - and observe where the butterfly is when closed and as you begin to move the throttle. If the progression holes are different, or if the butterflies are covering/uncovering them differently in each carb or throat, that would contribute to your problem.
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PostPost by: Stig » Sun Nov 28, 2021 11:32 am

Based on your feedback and also reading up on other posts on this forum and on the vintagetechnologygarage forum I have decided (I think) to replace my old Weber 45 DCOE 15/16 with new 40 DCOE 151.

A few more questions though:
-Can the 45`s be replaced by 40`s without changing anything on how they are attached to the engine, e.g. bolt CC distance, manifold dimensions, etc.?
-The standard set up of the 40 DCOE 151 I have seen is: Choke 30, Main jet 115, Main air correctors 200, Emulsion tubes F11, Idle jets 45F8, Accel pump jets 40, Needle valve 1.75, Auxilary venturis 4.5, Air intake trumpets 38mm.
-Are all this a good as a starting point before a possible fine tuning or should something be exchanged already from the beginning?

Furthermore when the carburettors are out anyway I am thinking of installing a 123Distributor as well. Is that advicable?

Any advice and comments are very welcome.

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