Weber advice

[gearbox]

I'm rebuilding a Weber TC motor for my 69 SE (originally a Federal Stromberg car). The engine has the big valves and an one groove SE type C Cams. I'm using new JE 20 over pistons and contemplating 1.75" headers, but that will be the extent of any mods. I also have a set of 40 DCOE's 151's that I am planning on rebuilding/using. Unfortunately, the PO had taken out the 4 emulsion tubes, holders, air correctors and main jets from the two carbs. The carbs currently has 30mm chokes. Given the application, I believe I need the following from the manual:

1) 32mm chokes
2) 115 main jets
3) 150 air corrector jets
4) F11 Emulsion tubes

I'm thinking it should be a good base line to start with, but since I will be buying all these parts new, is there any benefit of converting the carbs to the D Cam Sprint set up which has a 33mm chokes, 155 air correctors, and 120 main jets? I believe everything else is the same in the carb. As I had mentioned, the engine is a Big Valve, but has the C type Cam.

On a side note, I will also be needing the linkage and the interconnecting linkage between the two carbs that uses a barrel to tie onto the cable. On the interconnecting linkage, I have read on another post that there was a specific Lotus version that was shorter. Specifically the side with the barrel lock. The only ones I found available was this type that has the longer arm http://www.piercemanifolds.com/product_p/pm3719.htm Is there somewhere I can get the correct linkage? Does it matter? and if so, and the original is NLA, does anyone have the dimensions to mod the new version? As for the main linkage, which one works best? Top pull, bottom pull, etc. Thanks Allan

[gearbox]

This is the picture of the different interconnecting linkage I found on another post, the Lotus one is on the Left.

short%20lever.jpg (12.35 KiB) Viewed 3363 times

[Esprit2]

Contact Matthew Cooper at Eurocarb Ltd in the UK. He not only sells Weber and Dellorto parts, he's very knowledgeable about the Lotus applications.
http://www.dellorto.co.uk/

DCOE Lotus linkage lever
http://tinyurl.com/a4l8jlw
Part No - 45048.007 GB?12.35 ex. VAT

For cable retaining barrel see p/n
http://tinyurl.com/aq7o943

Regards,
Tim Engel

[gearbox]

Thanks for the leads Tim, I will contact them. However, I see the linkage they are selling, and it looks very simular to the standard one being sold here in the states. See my picture on the right. So I assume it doesn't matter if I use the shorter one? Thanks again Allan

[Esprit2]

Carbs, Cams, Compression... keep them in balance. Making a significant change in one without taking the others along won't deliver the full expected impact. Going to the Sprint carb specs without the D-cam and Sprint specs won't give you the same affect.

Having said that, the difference between the C & D cams and the small Vs big valves is pretty minor, and you don't mention any change to the compression ratio (9.5 ??). The Sprint carb set-up should probably work equally well on your engine, but there will be a change in the engine's personality. The question isn't so much a matter of whether either carb set-up can be tuned to work, but rather what you want the engine to run like once you put it into use.

The SE was a pretty street friendly engine, and the Sprint had to be pushed harder to deliver it's extra performance. Going from 30mm to 32mm chokes will push peak performance a higher up the rev range, and the increases will come at the expense of low & mid-range performance.

If you're going to drive the car in a spirited, but civilian manner, maybe commute to work with it, then the smaller, 30mm chokes are probably more appropriate. Even stock and street-prepared autocross generally puts more emphasis on mid-range torque and powering out of a myriad of corners than it does peak power. The smaller 30mm chokes would be more at home there.

On the other hand, if you plan to use the car for track days and will be spending a lot of time at the top end of the rev range, then the 32mm chokes might be a better way to go.

The difference between the overalll SE and Sprint specs is minor enough that I think the answer to your carb question really lies within you. How do you plan to drive the car most of the time? Carb accordingly. Just don't expect to carb for the top end and not pay some price in the low end.

Regards,
Tim Engel

[Esprit2]

Thanks for the leads Tim, I will contact them. However, I see the linkage they are selling, and it looks very simular to the standard one being sold here in the states. See my picture on the right. So I assume it doesn't matter if I use the shorter one? Thanks again Allan

Allan,
The throttle requires X-many degrees of rotation to reach WOT (Wide Open Throttle)... and you definitely want to reach WOT. It requires fewer linear inches of cable pull to deliver that rotation with a short lever arm, and more inches with a longer lever arm.

If the butterflies just reach wide open as the pedal bottoms out on the floor with the short Lotus lever, then you will be hard pressed to find the longer pedal stroke required to work with the longer generic linkage lever.

However, if the pedal currently uses an adjustable throttle-stop to stop it short of the floor at WOT, then you may have no problem freeing up some extra pedal stroke to make the longer lever work all the way to WOT. But your foot will have to push the pedal further to reach WOT.

In use, you'll have to use more pedal stroke with the long lever, and less stroke with the short lever. It's a bit of a placebo effect, but the engine will seem to respond more aggressively to small throttle inputs if the short lever is used.

The website only shows so much. Call or e-mail Matthew Cooper, tell him what you're doing with your Twin Cam, and ask about the available levers. He is a good resource... use him.

Regards,
Tim Engel

[gearbox]

Hi Tim, thank you again for your words of wisdom. The head nor block were cut, so I assume stock compression, 9.5:1? The car will be a road car with the occasional autocross and I like your idea of keeping the 30mm chokes as it saves me money on a project that is already way over budget. Pushing 90 mph on my other stock Elan is a real hoot, but doubt I will be spending much time there as the car feels a bit on the edge, plus I like having my license. But the Elan in question is fitted with new TTR Koni's, ride and height adjustable, and a Spyder roll bar and anti intrusion side bars. Not that the bars will have any bearing on handling, but it'll give me some confidence of surviving a hit. But my main concern would be reliablity and low to mid range. With that being said, should I keep the 30mm chokes, instead of the 32mm for SE cams I have, or even the 33mm for the Sprint? But balance is what I am trying to achieve. So with my SE cams, stock compression, Big valves, etc., will the 30mm chokes starve the engine and should I go to the 32mm? Like everyone else, I would like to get the max out of the motor without sacrificing drivability. Somewhat like the difference between the Extige and Elise. The Extige is a great car, but I perfer the Elise for the streets. So bottom line, if I stay with the 30mm chokes do I go with the B cam specs, or do I go with the 32mm chokes and stay with the C Cam specs? Or maybe the 30mm choke and the C cam specs for the emulsion tubes, air correctors, and main jets? I know I will be playing with all these when I get the car running, but I need a starting point. Thanks again Allan

[AHM]

Just about any set-up will do as a starting point. as Tim says each component gives different characteristics so change them to get more of what you want.

There is a good article somewhere here that explains what each part of the weber does - If you understand that you will understand what to change. Experimenting can be expensive, it may be worth a rolling road session.

For your C/R don't forget that you have gone up to +020

[gearbox]

I guess the question I am asking here is what set up should I start with that will be close to my desired end result without having to change everything later. I understand there will be some adjustments and changes down the road, but based on my engine configuration and my desire for a reliable set up focusing on the low and mid end how should I start? So what choke, emulsion tube, air corrector jets, and main jets do I need? I'm looking for a smooth idle and fast throttle response.

The engine is basically a Weber head SE with 20 over pistons. It has the big vavles, but given the nominal increase of the intake valve over standard SE, I don't think that will effect the engine that much if at all. Headers are the only other mod that will be done.

I was thinking starting with the factory SE set up using 32mm chokes and jets assuming the engine will be slightly better than SE Spec, but not as good as the Sprint Spec. Tim's suggestion of retaining my 30mm choke intrigues me, but do I use the factory spec for the B cam jets and tubes that goes with the 30mm chokes, or do I use the SE spec jets? I just don't know enough about webers and I was hoping someone on this forum had worked this out before me. Thanks Allan

[Esprit2]

Allan,
I'm exaggerating, I know, but... You're asking us to tell you which girl you should marry. That's really up to you, and a decision you have to make.

Your engine is near stock, and the three factory versions, Std, SE & Sprint, don't really differ all that much. It's NOT like only one of the three factory carb set-ups will be the right one, and the others won't work. Each will work in it's own way.

The modern ECU/ sensor controlled engines in the Elise and Exige can adapt to a variety of conditions on the fly. Carbs can't do that. With carbs, you tune for one set of conditions, and live with it... ya gotta choose. If the conditions change, or your priorities change, then you have to re-jet the carbs. With modern ECU controlled, fuel injected engines, you get to play with a lot of girls. With carbs, you must choose one.

As I tried to convey earlier, the smaller chokes will be more street friendly, with better throttle response, and a stronger low & mid-range performance. If that's the personality you want the car to have, then go with the smaller chokes.

Each of the larger-choke set-ups, 32mm & 33mm, will deliver incrementally more peak horsepower toward the top of the rev range... more of a balls-out, boy-racer personality. Bury your foot in it, shift gears a lot, and it will go faster. But off-idle throttle response, mid-range acceleration, and commuting in traffic might not be as pleasant.

The difference in cams between the Std, SE and Sprint versions is not dramatic... you can cover the differences with a dime. It's not like you're chosing between street and track. IMHO, any of the stock Lotus carb set-ups can be tuned to work with the Std, SE and Sprint engines.

Smaller chokes are better for comfortable street driving, and larger chokes are better for boy-racer, go-fast stuff. Pick where YOU WANT to be. Having made your choice, the stock Lotus carb jetting set-up for that choke size will get you into the ball park for first-start. Get the engine running, and then tune from there.

Do you want to buy carb parts just once, and have it all turn out perfect the first time? Good luck with that. If it happens, get your buns off to Vegas/ Atlantic City right away before your luck runs out. But other than that miracle happening, plan on tuning/ re-jetting after the engine is running.

"Disappointment is a function of expectations". Of the three stock Lotus carb set-ups for the Twin Cam, I can't tell you which one you will like the best. That all comes down to your expectations. The one choice you don't get with carbs is, "All of the above". Ya gotta pick one.

My engines all tend to be over-carbureted with hotter than stock cams. I like to run 'em hard, and I'm willing to put up with a little attitude in traffic. From what you've written, I don't think you would be every-day-happy with my cars. You need to choose, small, medium or large, and don't go by my priorities.

Since you're not sure, I would suggest that you rebuild the carbs per their current choke size (minimum expense... go with whatcha got). Get the engine running and drive it for a while. If you like it, great! However, if you find yourself always driving it with your foot on the floor and wishing for a stronger top end, then rebuild the carbs to a larger-choke spec later. That's easy to do later without any need to remove or dismantle the engine, and will only take the engine out of service for a short time.

Good luck,
Tim Engel

[gearbox]

Hi Tim;

I love your anologies, but at 56 and being married for over 30 years, my choice is not to get married lol. But all kidding aside, I gather from your comments that fitting a standard carb with the 30mm chokes will not cause any fuel starvation or poor performance for the SE engine I am building. Also since I am using the 30mm choke, I should stay with the factory B cam spec emulsion tubes, air correctors, and main jets, and not use the C type cam specs for the 32mm chokes. Yes? Seems like the only difference would be the Air corrector jets which are 200 for the B cam and 150 for the C cam. Sorry to belabour the point, but I want to be sure before ordering the parts. Thanks again for all your help Allan

[AHM]

Allan,

They will all work, but we can't tell you what you will like, or what will suit your engine.

You need to understand what each part does, and what effect your change will will have - Did you find the article I mentioned? When you ask about the air correction jet you need to know that a bigger number is leaner at higher RPM and understand why 200 suits the 30mm choke.

-Air and fuel have different densities so accelerate differently
-In a smaller diameter choke the air will have to move faster
-Faster moving air at low revs will give you better low down performance
-A larger diameter will allow more breating at the top end.

lotus will have done lots of testing so if you stick with a 'standard' set-up you wont be far off.

Simon

[gearbox]

Allan,

They will all work, but we can't tell you what you will like, or what will suit your engine.

You need to understand what each part does, and what effect your change will will have - Did you find the article I mentioned? When you ask about the air correction jet you need to know that a bigger number is leaner at higher RPM and understand why 200 suits the 30mm choke.

-Air and fuel have different densities so accelerate differently
-In a smaller diameter choke the air will have to move faster
-Faster moving air at low revs will give you better low down performance
-A larger diameter will allow more breating at the top end.

lotus will have done lots of testing so if you stick with a 'standard' set-up you wont be far off.

Simon

Thank you Simon, that was the answer I was looking for. I had searched extensively on weber set up, emulsion tubes, air correctors, etc., even before posting this question, but the post you mention still eludes me. I will keep on trying. You would think a insightful topic like that should have been loctaed sepreately under the techinical section so it was easier to access. But anyway, I now understand some of mechanics on how the 30mm choke works with the Air Correctors and will follow that direction, thank you for that. They say a "Little knowledge is a dangerious thing" and I think I am just getting to that point with Webers lol. If you come across that thread, let me know, I would love to read it and get into more trouble. But for now, I am ordering the B Cam set up. I'm not sure why Webers seems so complicated to me, but even trying to find the correct throttle linkage was challenging. I was looking for that female intercoupler linkage, the one with the two adjustment screws that connects the with the adjoining carb, just to find 4 different types, with 4 different prices, and 4 different part numbers. And the really strange thing was that the pictures all looked the same. Go figure. Thank you again Allan

[Esprit2]

Allan,

The adjustable linkage coupler connects the throttle shafts of two adjacent carbs, therefore it must be long enough to span the gap between the carbs. On a short engine, like the Twin Cam, the carbs are close together and use a short coupler. On a longer engine, like the Lotus 907, the carbs are further apart and need a longer coupler. The couplers are available for different engine applications, which is why you find listings for couplers that "look" the same, but have different part numbers.

The adjustable balance coupler was a Lotus development that was then copied for many applications on both Webers and Dellortos. Some couplers use two opposed screws, and you must back one screw out before you can turn the other screw in. The Lotus coupler used a single screw working against a spring-loaded plunger... easy single screw adjustment. The convenience makes it worth looking for the single screw variety.

You don't have to know the differences between every coupler made, you just need to order a coupler for two 40DCOE carbs on a Lotus Twin Cam. Once again, contact Matthew Cooper at Eurocarb, tell him what you need, and he'll take care of you.

*~*~*
Please get away from referring to the different carb set-ups by cam association. It's confusing.

> But for now, I am ordering the B Cam set up.

Throughout the early part of this thread, I thought you had a stack of cams available, and you were flirting back and forth between which cam to use when building the engine. It took a while to figure out that the engine's basic build-up is set, and you were saying you were going to use the carb set-up that Lotus used in the Standard Twin Cam that was also equipped with the B-cam. That's a pretty distant, loose association, and most Weber parts dealers you talk to won't have a clue what you're saying.

Identify the Lotus carb configurations by the choke size and it will be easier for others to follow what you're doing.

Try, "For now, I am ordering the 30mm choke set up".
Cool, now I know what you're doing.

*~*~*
The Weber DCOE (and Dellorto DHLA) has two circuits, Idle and Main.

Vacuum in the Idle circuit is generated by the engine sucking against a closed throttle butterfly. It has all the sophistication of sucking on a soda straw. At closed-throttle idle, the vacuum is strong. Then as the butterflies open, progressively exposing the throat to atmospheric pressure, the vacuum drops off. By some mid-throttle point, the circuit is no longer very effective.

Vacuum in the Main circuit is generated by air flow velocity through a venturi. At idle, air flow is minimal, and there's not enough flow velocity through the venturi to lift any fuel up out of the float bowl... it doesn't work at idle and small throttle openings. As the throttle is opened and air flow increases, the venturi vacuum also increases until it starts drawing fuel into the throat.

The idle circuit gets weaker as the venturi main circuit gets stronger, and at some transition point, there's a hand-off between the Idle and Main circuits. In Webers, the transition point is around 4000-4500 rpm. In Dellortos, it's around 3200 rpm. IMHO, sucking on a soda straw (Idle circuit) is pretty crude and doesn't provide much resolution for the control of the Air Fuel Ratio (AFR). Compared to Weber DCOEs, the similar, but more refined Dellorto DHLAs are much better in that regard.

The emulsion tube well is a vertical, cylindrical hole in the carb body... usually located in a pedestal in the middle of the float bowl. Part way up the bore, there are intersecting holes drilled. Vacuum lifts fuel from the float bowl up in the well... like that soda straw, again... until it spills over into the cross drillings and flows down into the carb throats.

The height of the cross drilling determines how much vacuum is required... how high the fuel must be lifted... before it spills over into the throats. The carb designer can set that height as he sees fit. In the Weber, the spill-over height is higher than in the Dellorto, meaning the venturi must be sucking harder before it actually draws fuel into the throat, and the Weber's transition point occurs at a higher rpm. Make sense?

A venturi is a necked-down restriction is a passageway. If you put a 30mm choke in a 40mm throat, you've created a simple venturi. The greater the neck-down, the more pronounced the venturi effect, and the stronger the vacuum generated at any given flow rate. A 30mm choke in a 40mm throat will generate a stronger vacuum than a 33mm choke will (larger choke bore = less neck down = weaker vacuum).

When you change choke sizes, you change the overall venturi vacuum strength in the carb. The strong vacuum of a 30mm choke will really pull the fuel over. To dial the fuel flow rate down to a correct fuel mixture for the engine, some size of main jet will be used.

If you switch to larger 33mm chokes, then the venturi neck-down is reduced, the venturi effect is less pronounced, and there's less vacuum available to pull fuel over from the float bowl. With less vacuum and less fuel flow at a given air flow rate (rpm), a larger jet will be required to flow the same, appropriate amount of fuel.

The main jetting set-up is only appropriate relative to the venturi vacuum level present in the throat, and choke size has an overall impact on that venturi vacuum. Most carbs will also have an auxiliary venturi, but the choke IS the primary venturi. For a given carb size (40, 45 throats), the jetting/ air correction/ float weight & height... anything that affects mixture... has to be entirely re-thought for each choke size. Choke size screws around with everything else.

Hopefully you can see now that it would be totally inappropriate to take the jetting from a 33mm choke carb on a Sprint engine, and install it into the 30mm choke carb on a Std engine. With a stronger overall vacuum condition in the 30-choke carb, the 33-choke jetting would be impossibly rich. Make sense?

*~*~*
There are separate emulsion tube wells for the main and idle circuits. The idea is the same in the idle wells, except it's the engine sucking against the closed throttle butterflies that produces the vacuum to lift the fuel up to the cross drillings... ie, the spill-over height.

The Idle circuit feeds the engine's fuel needs up to the transition point... not just at idle. The Main circuit then takes over, and feeds the engine up to the top of the rpm range.

BIG chokes provide less restriction in the carb throat, so they flow more air and hence, have more top-end power potential. However, they also provide less venturi neck-down and are less effective at producing vacuum. Hence, they're not as good at delivering fuel at lower air flow rates (mid-range rpm), and they don't have the positive, right-now affect on fuel flow rates in response to throttle changes (fuel is heavy, has more inertia than air)... ie, throttle response isn't as immediate. Smaller chokes are better in the constantly varying conditions of traffic, larger throats are better on the relatively constant wide open throttle conditions on the track.

That's a really simplified version of how fixed-venturi carbs work, and it leaves out a ton of detail; but hopefully it gives you a mental image of what's going on. Constant Depression carbs, like SU and Zenith-Stromberg, are a different kettle of fish, and require a different understanding.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[AHM]

I'm no expert!

I can't find it either maybe it isn't there! Try this one instead!
http://www.tr3a.info/WeberDCOEinfo.htm

And for reassurance this
elan-f15/which-use-dcoe-dcoe-151-t1799.html

Before you buy anything. Post in the wanted section and look on ebay etc the standard set-up is what everyone changes in the persuit of power.

To follow on from Tim's analogy..
If she is built for comfort the top end performance won't be there
If she is built for speed she will be fussy and dificult to live with on tickover

Are the 151's the right carbs for an elan?