Lotus Elan

Do weber 40 dcoe,s need a choke ?

PostPost by: Concrete-crusher » Mon Jun 30, 2014 12:42 pm

Hi I,m in the process of replacing my elan dashboard , and am fitting the mechanical parts first. My webers have the choke cable disconnected and it's always seemed ok.

So do webers benefit from the choke cable being fitted , when cold starting ?

Or should I leave mine off.

Steve
Concrete-crusher
Third Gear
Third Gear
 
Posts: 340
Joined: 09 Jun 2013
Location: UK

PostPost by: MickG » Mon Jun 30, 2014 12:56 pm

Why not just fit it and not use it :D
Last edited by MickG on Mon Jun 30, 2014 6:16 pm, edited 1 time in total.
MickG
Third Gear
Third Gear
 
Posts: 314
Joined: 14 Jan 2011
Location: Essex,UK

PostPost by: Concrete-crusher » Mon Jun 30, 2014 1:02 pm

My wife would ask why I have an elan for over twenty years and not drive it. I just seem to be constantly taking bits off and cleaning or fixing things.

In between houses and kids that is.
Concrete-crusher
Third Gear
Third Gear
 
Posts: 340
Joined: 09 Jun 2013
Location: UK

PostPost by: Esprit2 » Mon Jun 30, 2014 5:26 pm

You've kind of answered your own question:
> My webers have the choke cable disconnected and it's always seemed ok.

No carb 'needs' a choke, but starting the engine cold is easier with a choke. Doing so without the choke just requires more user knowledge and involvement... something you seem to have worked out.

I don't have the choke cables hooked up on any of my Weber or Dellorto carbed engines, I live in a northern climate, and I drive them as deep into the Winter season as I can... parking them only when the salt goes on the roads. They start.

But consider the complete car. "IF" you're interested in preserving some degree of originality, why not install the choke cable. Doing so would look better than having a blank spot on the dash. It's a minor project, and nobody says you have to use it if it's there.

Regards,
Tim Engel
Esprit2
Third Gear
Third Gear
 
Posts: 325
Joined: 02 Apr 2008
Location: Twin Cities, Minnesota, USA

PostPost by: bill308 » Mon Jun 30, 2014 9:59 pm

I have 2-Weber equipped cars, a 1978 Ferrari GTS (4x40DCNF's) and a 1966 S2 Elan (2xWeber 40DCOE's).

I think use of a choke is a more elegant way of keeping the engine running after a cold start in cold conditions. The alternative is to pump or blip the accelerator until an idle can be sustained.

Conventional wisdom is to use the accelerator in lieu of the choke. I prefer use of the choke. One must be careful not to flood the engine though.

Bill
bill308
Fourth Gear
Fourth Gear
 
Posts: 775
Joined: 27 May 2004
Location: Windsor, CT USA

PostPost by: billwill » Tue Jul 01, 2014 12:47 am

'tis said that you don't need to use the choke on a Weber twin cam, but you do on a Dellorto Twin cam

Judging by the comments I've read on this forum over the years, it seems to depend on the driver and on the car setup, so there's no real way of knowing.

:D

I do use the choke on my car; it has Dellortos.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4523
Joined: 19 Apr 2008
Location: London UK

PostPost by: ftsoft » Tue Jul 01, 2014 11:56 am

I guess it depends on where you live or where you might be planning to drive. I've owned my S2 for almost 50 years, 45 of it in the US East of the Mississippi and I use the choke in Spring, Fall and Winter.

Frank
66 S2 26/5194
LP4711LA-B
User avatar
ftsoft
Second Gear
Second Gear
 
Posts: 179
Joined: 15 Sep 2009
Location: Ohio, USA

PostPost by: Esprit2 » Tue Jul 01, 2014 2:08 pm

Which is an operational choice on your part (and a perfectly valid one), not an operational 'need' of the carbs or the engine.

The original question asked:
"So do webers benefit from the choke cable being fitted , when cold starting? Or should I leave mine off."

Yes, they benefit in terms of ease of starting, but the difference isn't between easy and darned difficult/ don't work. It's more like more easy verses less easy. It's definitely not a matter of the engine won't start without the choke.

If you like what the choke does for ease of starting and running during warm-up, install it.

If you're indifferent to the cold start benefit, don't be concerned about the engine needing it... it doesn't. Install the choke cable if you want it (originality ?), or don't if you think it's a bother. No big deal either way.

Regards,
Tim Engel
Esprit2
Third Gear
Third Gear
 
Posts: 325
Joined: 02 Apr 2008
Location: Twin Cities, Minnesota, USA

PostPost by: elanner » Tue Jul 01, 2014 4:02 pm

Tim,

I recently broke down and read my Haynes Weber manual for the first time. I now realize that the choke isn't a choke. It's a baby carburetor designed just for starting, and entirely separate from the main carburetor. The knob on the dashboard doesn't choke anything, it simply turns the starter carb on.

So it seems to me that when you are starting a cold engine you decide either (1) to use the main carburetor, using fine control of the accelerator pump and pedal, or (2) to pull the knob and use the starter carburetor.

Presumably it's counter-productive to use both carburetors together, so don't do (1) and (2) at the same time (which is what I typically did).

All the pictures that I've seen of the starter in operation show the throttle butterfly valve closed. The starter has its own jet and air inlet and doesn't appear to need any help from the main carburetor.

So for the past few weeks I've been using just the starter carb, not touching the pedal. It's amazing how counter intuitive this is to do! Nevertheless I've found that the car starts very well and is a lot more predictable about it.

I'm guessing that (1) emulsified petrol from the starter carb makes for easier starting than neat petrol from the accelerator pump, (2) the starter design creates an air/fuel ratio that is appropriate for a cold engine, and (3) fiddling with the accelerator pump and pedal at the same time screws up the starter carb and risks flooding.

Is this right? So far it seems to work well and make sense (to me, at least). :-) But I've never seen it described anywhere so perhaps I'm just blowing smoke. Or perhaps everybody but me already knows. :-( Of course the acid test will have to wait for some cold weather.

When cleaning my carbs I found the small wire mesh that covers the air inlet for the starter carb to be quite clogged up. Presumably this effects the air/fuel ratio but it's easy enough to remove and clean.

So I vote for keeping the "choke" dashboard knob - and also to try using it, but alone!

Nick
User avatar
elanner
Third Gear
Third Gear
 
Posts: 477
Joined: 14 Sep 2010
Location: Boston, MA, USA

PostPost by: billwill » Tue Jul 01, 2014 4:43 pm

I was just about to say it isn't really a choke anyway, but you beat me to it and yours is an excellent explanation.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4523
Joined: 19 Apr 2008
Location: London UK

PostPost by: Esprit2 » Tue Jul 01, 2014 4:56 pm

Nick,

True, neither the Weber DCOE nor the Dellorto DHLA have a choke. A choke is a secondary butterfly that closed at the inlet side of the throat, applying an additional vacuum across the overall system to pull additional fuel across from the float bowl.

The DCOE/ DHLA and Zenith-Strombergs use an "enrichment device" that adds extra fuel to the mix without the need for a supplemental butterfly. It's just a different way of accomplishing the same thing.

Calling it a "choke" is like calling every photo copier a Xerox, or every facial tissue a Kleenex. "Everybody" does it.

*~*~*
You don't have a choice between the main carburetor and the enrichment carburetor, as you said. The DCOE/ DHLA "main carburetor" has two circuits, idle and main, which hand-off the fuel metering duties between them in a designed-in manner. That is there all the time, whether you pull the "choke" on or not.

The "enrichment device" is supplemental, metering in an additional amount of fuel to provide an overall richer mixture as required by the driver's judgement. It isn't a separate, parallel carburetor, and you aren't choosing between two.

All the Air/Fuel mixture delivered to the carb throats by the Idle Circuit, the Main Circuit, or the Enrichment Device is a metered emulsion.

Cold starting without the enrichment device isn't a matter of keeping the engine alive with shots of raw gas from the accelerator pump... at least if you do it right. Once you kick the engine's idle rpm up a bit with your foot, and hold it there, the steady-state operation runs well beyond the accel pump's shot duration, and the engine is living off a metered emulsion from the Idle Circuit.

If, instead, you elect to use the Enrichment Device, the Idle Circuit will continue to do it's thing, and pulling on the "choke" cable doesn't disable that. All you're doing is activating a supplemental fuel metering mechanism that can take a variety of forms, and adding another dose of fuel in additional that already provided by the carb's Idle Circuit. You're double-dipping.

> Presumably it's counter-productive to use both carburetors together,
> so don't do (1) and (2) at the same time (which is what I typically did).

As noted above, you don't have such a choice. The carburetor's base function is there all the time whether the engine is cold or hot. And at low rpm (below 4000 rpm for Weber and below 3200 rpm for Dellorto), it's the Idle Circuit that is feeding the engine's basic needs.

Pulling the "choke" to start the enrichment circuilt does not make a choice between the main carb and the choke carb. It only adds an additional amount of fuel from the enrichment device. Frosting on the cake.

> All the pictures that I've seen of the starter in operation show the
> throttle butterfly valve closed. The starter has its own jet and air
> inlet and doesn't appear to need any help from the main carburetor.

At no time when the engine is running is the butterflly truly closed... some air must be admitted. At idle, the butterfly is 'nearly' closed, and that's what the illustration shows. The engine is sown idling with the butterfly nearly closed, and supplemental enrichment being provided by the enrichment device/ circuit. Everything else that's going on in the carb isn't illustrated because the focus is upon the Enrichment Device.

> I'm guessing that
> (1) emulsified petrol from the starter carb makes for easier starting than neat
> petrol from the accelerator pump,
> (2) the starter design creates an air/fuel ratio that is appropriate for a cold engine, and
> (3) fiddling with the accelerator pump and pedal at the same time screws up the starter
> carb and risks flooding.
>
> Is this right?

Emulsified fuel metered through the Idle Circuit, Main Circuit, or Enrichment Device does work better than a solid slug of raw fuel from the accelerator pump. But starting and warm-up without the enrichment device does not depend solely upon the accelerator pump. That's where your understanding sort of derails.

The enrichment device adds an additional amount of pre-mixed emulsion in response to the driver's demand. It does not monitor or control mixture in anyway, a the driver can ask for too much or not enough enrichment, and get it. It's not a "smart" circuit.

Again, fiddling with the throttle/ accelerator pump while also using the enrichment device can produce an over-rich condition. Carburetors are dump, there is no computer/ O2 senser feedback loop, and the driver must know more about what he's doing than the driver of a modern ECU controlled fuel injected engine must know. Carburetors require the driver to be a knowledgeable participant in the process, and that's a skill that has been atrophying in motoring society since fuel injection became dominant in around 1985. Prior to that, it was everyday life, if you owned and drove a car.

> But I've never seen it described anywhere so perhaps I'm just blowing smoke.

Well, maybe not blowing smoke, but gas fumes at least.

> When cleaning my carbs I found the small wire mesh that covers the air inlet for
> the starter carb to be quite clogged up. Presumably this effects the air/fuel ratio
> but it's easy enough to remove and clean.

Yes, keeping that screen clean is as important to proper carb function as is keeping the main air filter clean or replaced. But out of sight is out of mind, and most owners ignore the screen.

Regards,
Tim Engel
Last edited by Esprit2 on Thu Jun 06, 2019 8:29 pm, edited 2 times in total.
Esprit2
Third Gear
Third Gear
 
Posts: 325
Joined: 02 Apr 2008
Location: Twin Cities, Minnesota, USA

PostPost by: elanner » Tue Jul 01, 2014 8:47 pm

Tim,

That's very helpful, thanks.

You're right, I over-emphasized the point about the two carbs being unrelated - I prefer your "enrichment device" terminology. I realized that the throttle butterfly valve is never fully closed, so the idle jet must play a role in starting even when you don't touch the accelerator pedal. I vaguely assumed that the engine RPM while on the starter motor is so low that there isn't a lot of vacuum (especially when air is being drawn through the enrichment device) so the role of the idle jet would be incidental. Meanwhile, the advantage of the pump, of course, is that it doesn't need any vacuum at all.

Anyway, this seemingly obvious topic turns out to be very interesting. My engine is starting well using just the enrichment device right now, but I'm continuing to experiment. I suspect that it'll be a different story in Winter - and I'll be back to the standard fine touch that we all develop!

Nick
User avatar
elanner
Third Gear
Third Gear
 
Posts: 477
Joined: 14 Sep 2010
Location: Boston, MA, USA

PostPost by: Esprit2 » Tue Jul 01, 2014 9:25 pm

elanner wrote:(Snip)... I vaguely assumed that the engine RPM while on the starter motor is so low that there isn't a lot of vacuum (especially when air is being drawn through the enrichment device) so the role of the idle jet would be incidental. Meanwhile, the advantage of the pump, of course, is that it doesn't need any vacuum at all.
Nick,
Venturi type carbs like the Weber and Dellorto have two basic circuits... the Idle Circuit and the Main Circuit (Constant Depression carbs like SU and Stromberg are a different can of worms).

At 'closed' throttle and small throttle openings, the engine is sucking against a 'closed' plate, which is a pretty effective way to create a vacuum. As the throttle opens more, the throat is exposed more and more to atmospheric pressure, and the vacuum weakens. You can't suck a strong vacuum against a closed plate when the plate is open.

However, by the time that vacuum is weakening, air flow velocity through the throat has also picked up, and the venturi is creating more and more vacuum. At some point, the rising venturi vacuum exceeds the falling sucking-against-a-plate vacuum, and takes over the carb's operation.

Sucking vacuum drives the idle circuit. Venturi vacuum drives the main circuit. The height of the fuel level (float setting), the fixed lift-over height, and the strength of the available vacuum determines the transition rpm between the idle and main circuits.

In a Dellorto DHLA, the designer selected a lift over height that results in a transition point at about 3200 rpm (if all other guidelines were followed in matching a carb to an engine). The idle circuit feeds the engine up to ~3200 rpm, at which point the venturi-fed main circuit takes over and feeds the engine up to red line.

In a Weber DCOE, different dimensional design choices were made, and the transition point is closer to 4000 rpm. It just is what it is. You can agree with a carb's design specs, or you can buy another brand of carb, but designed-in/ machined-in features are not adjustable. I like Dellorto DHLAs more than Weber DCOE's for a number of reasons, but mostly for their basic design architecture on a street engine.

The above design features are the designer's tools and choices.

Throat and choke sizes for a given engine size also affect vacuum strength, and can also have an impact upon vacuum strength at engine rpm, and upon transition point; but these are the engine builder's/ carb tuner's choices.

*~*~*
For starting, the throttle is (nearly) closed, so the engine can draw a good, strong vacuum even at cranking speed. And it's the Idle Circuit, working on 'sucking' vacuum that feeds the engine during starting and up to the transition rpm.

The Idle Circuit and the Enrichment Device are both driven by 'sucking' vacuum. So when the 'choke' is pulled on, everything in the Idle Circuit continues as before, and the Enrichment Device kicks in to add and additional amount of fuel to the same airflow, creating a richer mixture. For the most part, it meters like a faucet... the more you pull it on, the more fuel is dispenses. It's not smart... at least no smarter than the operator. It's just a pre-set orifice and a vacuum source peeing additional fuel into the air stream. And it works as long as the 'sucking' vacuum supply is strong... ie, and small throttle openings.

Regards,
Tim Engel

PS...
I know, 'sucking' vacuum isn't a technical term, but it creates that mental image that helps with understanding how the carb works at low throttle settings.
Esprit2
Third Gear
Third Gear
 
Posts: 325
Joined: 02 Apr 2008
Location: Twin Cities, Minnesota, USA

PostPost by: rgh0 » Wed Jul 02, 2014 12:09 am

Just thought I would add my 2 cents / pence worth to the topic.

In general the twin cam ( and similar engine carb designs such as the Lotus 907) dont need the choke / enrichment device for starting or cold running and can be started with a couple of pumps of the throttle and will then start and run fine in cold weather.

This is possible because

1. The simple straight and down flowing inlets ensure the fuel reaches the cylinders when the engine is cold rather than pool in the bottom of the manifold which is common in a typical single carb with branching manifold designs.

2. A couple of shots from the accelerator pump puts enough fuel into the cylinders that suffiicent evaporates for the engine to start and run even when its cold.

3. The "normal" idle mixture on a twin cam is pretty rich and once the engine is running sufficent evaporates even when cold to give a reasonable mixture while the engine heats up. The carb venturi per cylinder and resultant strong suction pulse and simple straight inlets helps this to happen.

The choke can be used to add fuel to the idle mixture but you still may need a couple of accelerator pump shots to get suffient fuel for the initial start in cold weather.


My plus 2 did not need the choke and I have run it on both Dellortos and Webers. When I changed the jetting to get a leaner idle circuit mixture for better fuel consumption and better transition to the main circuit using Keith from Sidedraft_central jets and emulsion tubes I found I needed to use the choke a little or fully warm the engine to smoothly get up my very steep drive way on a cold day

cheers
Rohan
Last edited by rgh0 on Wed Jul 02, 2014 12:59 am, edited 1 time in total.
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7077
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: Esprit2 » Wed Jul 02, 2014 12:53 am

rgh0 wrote:2. A couple of shots from the accelerator pump puts enough fuel into the cylinders that sufficent evaporates for the engine to start and run even when its cold.
That's really the key point to starting a cold engine without the choke. Get in and give the throttle a few strokes from full up to the floor. Then let it sit and evaporate while you belt-in, adjust the mirror, anything else you need to do (longer is better).

Then clutch to the floor, crack the throttle open about half an inch at the pedal (very non-critical), and turn the key. When the engine fires, catch it with the throttle pedal, and kick it up to a modestly fast idle and hold it there. Not pumping the throttle, since you're not trying to feed it with the accelerator pump... that part is done. Just a minimally fast idle that will keep the engine alive.

If the engine fast idles for a short while, then starts to slow and stutter like it might be about to quit, blip it up with the throttle again and re-establish a new idle. Never give it a full kick of the throttle or use high revs when the engine is stone cold. Just a blip of the throttle to poke the engine awake, rather than kick it.

Once it settles down to a smooth idle, you can give it a blip of the throttle to clear it's throat, and help it get off the line. Drive it gently when stone cold, without high revs. If you have the right oil in the engine for the season, warming the engine up while driving gently is no worse for it than letting it idle 'til warm.

Regards,
Tim Engel
Last edited by Esprit2 on Wed Jul 02, 2014 2:25 pm, edited 2 times in total.
Esprit2
Third Gear
Third Gear
 
Posts: 325
Joined: 02 Apr 2008
Location: Twin Cities, Minnesota, USA
Next

Total Online:

Users browsing this forum: No registered users and 5 guests