Dual Weber 40 DCOE tuning

[TheBrownHornet]

Greetings Ladies and Gents,

I'm the proud owner of a Series II. She has been fairly modified and was campaigned in historic racing in the early noughties (up to 2003?)

She has had a heart transplant with what I'm led to believe is a 1725 Holbay engine (or at least Holbay spec). I haven't had her apart to determine exactly what internals, cam specs, etc are involved. The distributor has been modifed and I assume remapped, vacuum advance removed but is still running points.

It came with a set of 40DCOE webers fitted. They work a treat when I've got my foot to the floor and the engine spins happily past 6000, but for low speed cruising with corresponding slight throttle openings she hunts and misses a little. I assume at a 60kph cruise, I'm only running on the progression circuit, thus need to be playing with my idle jet and air bleed to find the correct mixture.

I've read as extensively as possible regards tuning these things, but there still seems to be a fair amount of trial and error involved.

My request to any of you out there running DCOE's is to post your setup, that I may compare to my own to try to get the low speed end of the carbs correctly tuned.

Here's what I'm running at the moment.

Twin 40DCOE Weber
Chokes 36mm
Auxiliary Venturi 4.5mm (pretty much the standard from what I know)
Floats set at 8mm and 15mm as per Weber manual

Main Jet 175
Emulsion Tube F16
Main Air Bleed 200

Accelerator jet 35
Accelerator pump spill 50
Accelerator pump stroke 13mm (most documentation seems to recommend 11mm but I measured 13mm. Don't even know how to change it)

Idle Jet/Air Bleed 55F9 (I had 50F8 fitted but changed to the 55F9 to try to eliminate the low speed missing. It helped a little but didn't eliminate it)

For anyone still with me, apologies about the lengthy post and thanks for perservering.

I look forward to your input.

Regards

Graham

 

[RootesRacer]

You are running 36mm chokes in a 40?

Good luck with that.


I would never go beyond 32 on 40s, if the engine actually needs that kind of flow then you'd best be getting some 45s, which the 36 chokes are IMO at the limit for too.

With the correct chokes you should not need a main jet beyond 150, its best to keep them below 140 if possible in order for the progression to work properly.

I imagine this setup has quite a flat spot.

Another example where bigger does not better make.

 

[alpine_64]

Ill post what i can remember at this point.. will be able to look it up tomorrow:

I have holbay 40DCOE's.. they run the 4.5 aux venturis.. are your webers 34/45 or 91/92 (in the numbers up the top) if so they are holbay webers, they also dont have the hole in the vertical face of the carb. The holbay cars ran the extended venturis to avoid drawing in fuel vapor from the pulsing of the rootes motor.

mains 115
idle 45f9
pump 35
chokes 30's
airs 175
etubes f16

guys here running DCOES on 2120 stroker allpine motors still run with 30 chokes and 120 mains...

your car might be a racer and set up to run at high RPM all the time and not to be driven on streets where the lower rpm matter (you say spins easily past 6.. what are you revving to? beyond 7 and you are brave).. but as jarrid said.. they are big chokes.. you would be better going to a smaller choke and getting the air to move a bit faster. what type of manifold are you running? what kind ofexhaust.. the motor would want to breathe very well for thos specs and also as a note the holbay head ran smaller ports on the intake with a tapered manifold.. even the e128 cammed hot motor only ran 30 chokes in its dcoes.


hope this helps.

 

[Jim E]

Always wanted to try even smaller chokes when I was running DCOEs to see if it helped with low speed throttle responce or the hole. Those chokes seem huge does the engine patch at idle?

One thing I noticed when Micheal was tuning his webers was how much smaller a main he was running compared to what my engine required. Not sure if this was due to better fuel down under than here in the US, smaller engine size or my fuel pressure was not what it should have been.

This is with a set of the factory Holbay Webers
30 or 32 chokes did not make a lot of difference other than the turns on the idles and moved the hole around a bit
F16 e tubes the magic e tube [why is that jarrid?] But Ian's car ran fine on F11s So?????
125 or 140 mains again not a ton of difference think I stuck with the 140
45 f8 or f9 idles I tried every idle jet in the box and we have a big box of jets and it came down to either one of these
200 air tried several differet air jets with not a heck of a lot of effect overall
ran with and without the extended 4.5 inch gizmos again it made little over all difference.

Keep in mind when I say little difference I was tring to fine tune remove the flat spot or hole. Mostly the jet changes moved the RPM the hole was at, changed the performance at WOT and had an effect on mileage but the car was very drivable other than a small flat spot.

While I am sure you know this, tuning is imposible if the carbs are not all sucking the same or very near the same and the throttle is not pulling equal on both carbs.

 

[RootesRacer]

Here is a short course on carb theory, these are not from a book:

1) Carbs work due to pressure difference across jets (idle and mains).

2) The main mixture (not idle) is determined by the depression at the venturi location in the choke, which puts depression on the main jet.

3) The size of the choke (venturi minor diameter) and the flow through the choke determines the depression as a function of airflow volume.

4) The main circuit is made up of the main jet, emulsion tube, air corrector and fuel level.

5) The main circuit is influenced on the size and type of all components, the emulsion configuration the fuel level (sets the main pressure differential) and the choke airflow product.

6) The purpose of the emulsion + air corrector is to allow air to emulsify the fuel at low to medium RPMs which allows a linearization adjustment to the mid RPM/load band.

7) The idle jet takes care of closed throttle fueling and is done by restricting the idle jet fuel flow via a needle valve. The idle jet is less important to setting idle mixture than the mixture adjustment is since it is adjustable and a large range of idle jets will still allow a quality idle.

8) The idle jet also provides a transition mixture adjustment since beneath the throttle plates there are progression holes drilled to allow an increasing amount of fuel to be delivered as a function of throttle angle for the first 15 or so degrees. It is for this reason that the idle jet MUST be selected based on the transition/progression requirement, and not based on idle quality or what you buddy says you should use. Different carb models have different progression hole sizes, different number of holes and different spacings, and therefore no particular idle jet is an end all when there are so many OEM specific carbs (including the H120 specific model).


Now that we have all this behind us, the important things to consider here is that proper carb tuning is getting each of these items dialed in independently from one another, and in the correct order.

The greatest mistake made in tuning carbs is in thinking that bigger is better.

Bigger chokes and main jets are bragging rights, and nothing else.
Whats important is getting the right parts in the carb to make it work.
A properly setup weber will make more power with smaller chokes and mains than one with larger even though the flow numbers are better, this is becuase getting high flow numbers with lower flow velocities and poor atomization is not the best way to burn fuel, and thats what its all about BURNING FUEL.

Now on emulsion tubes Jim.
F16 and F11 tubes are similar in function, they are the most common becuase its about all you can get these days. A good supply of other tubes would surely make the tuning experience better, but the emulsion is the magic bullet in making a compromised setup work reasonably well. I say compromised meaning that the right emulsion tube can make an otherwise bad set of parts work as if it done right. This is the case on really ragged edge tuned engines with oddball induction anomalies where they can work fine at high RPM/load but fall on their faces at low ro mid RPM/load. Its the tubes that allow reasonable introduction of main fuel to the aux choke when the main circuit depression is less than ideal. Now if you setup your carbs correctly and have proper main depressions then setting up the emulsion tubes becomes a lot less critical.

Back when I tuned webers regularly, the procedure I used (which came from the gurus at Redline Weber) is:

1) to pull the main circuit out of the carbs before you begin.

2) set all the idle mixtures to the same positions.

3) set fuel level by eye with a dipstick down the main wells (this is particularly important on alpines due to the downward intake angle effecting the factory prescribed dimensions).

4) start the car, adjust for reasonable idle mixture and speed, making sure the carbs are synchronized.

5) slowly rev the engine by opening the throttle till it begins to fall on its face when the throttle plates pass the progression holes. Adjust the idle jets till the engine revs smooth in that area and stays slightly rich (CO monitored, an AFR meter used or by "calibrated" exh. smell).

6) Install the main circuits.

7) Using an AFR meter, while driving, change main jets till you get about 13.5 AFR cruising, and 12 AFR under power. If the main jet gets larger than 125, drop 2mm on the chokes, try again.

8) The emulsion is influenced by the air corrector jet. The larger the air corrector, the earlier that emulsion begins to occur, which will richen the area between progression and the early main operating area. I would sue a 180 air corrector, the allowable range should be 150 to 200, though if you get the choke size and main depression set well, the air corrector and emulsion become rather unimportant since the main circuit will come into play much earlier.



If you do these things, most of you will end up with 27 or 28mm chokes, and you will have very drivable cars.

Ive recommended 27 or 28mm chokes to lots of people but either they never get back to me (common round here), or they do their own thing and whine about how the carb works till they decide to put a 2bbl on it (which BTW typically has 24 pri and 27mm sec chokes, or about 33% more effective depression as a function of equivalent air flow compared to 4, 27mm chokes).


Ok, too long my response.
Good luck.

 

[serIIalpine]

By the way Jarrid I put the 28mm chokes in my 40DCOEs that are on my 1612cc engine and they work very nicely. Thank you very much.

I have the other jets written down somewhere. I'll have to find them.

Eric

'62 SerII
1612cc 3 bearing engine, Isky SB2 fast road cam and Vizzard head mod cc'd at 38 cc

 

[Jim E]

Yeah I really wanted to try 27s or 28s but could not find a used set and could not make myself pay for a new set. I know the smaller the chokes from what I had on hand the better everything seemed to get, overall.

 

[TheBrownHornet]

Good Morning One and All (Morning here in West Aus at least)

Thank you all so very much for your detailed replies and suggestions.

The common theme here seems to be that 36mm chokes are just too big. I'll look at getting some 30's or 32's depending on what I can source over here. Is there anyone that can pass on a Western Australian or even australian source for second hand parts?

...if the engine actually needs that kind of flow then you'd best be getting some 45s.

I actually have a set of 45's on one of my other toys, but at this stage I think I'll leave them there. I can't imagine that I have managed to aquire the only Alpine in existence that requires 45's with 36mm chokes to breath properly.

are your webers 34/45 or 91/92
holbay cars ran the extended venturis

In that case mine are definitely NOT Holbay Webers. No extended venturi, hole in the front face and 40DCOE73 4L06 codes. I'm interested in these extended venturis. Are they the ones pictured in the photo gallery?

what type of manifold are you running? what kind of exhaust.

It's a Warnerford intake manifold. Not sure of the port sizes. One of these days I'll pull off the rocker cover and look for the castings to ID a genuine Holbay head or a standard.
Exaust looks like a custom. 4-2-1 with fairly long pipes. Other than that I don't know anything more about it.
RPM. Just for the record and as this is more of a daily than a racer at the moment I generally keep well below 6k with only the odd blast around there to blow the cobwebs out.

One thing I noticed when Micheal was tuning his webers was how much smaller a main he was running compared to what my engine required. Not sure if this was due to better fuel down under than here in the US

Interesting. I feed her premium unleaded which over here is 95 octane. What fuel do you guys run in the US?

tuning is imposible if the carbs are not all sucking the same or very near the same and the throttle is not pulling equal on both carbs.

Forgot to mention that I have just been through a full rebuild/clean of both carbs. There was a little scale, but not much crap because I run a fuel filter.
Also replaced the soft mounts which I believed fixed a vacuum leak. I have synchronised butterflies by looking down the progression holes and adjusting, and then balanced air flows using the air correction/bypass screws. Just used the synchrometer type thing on the ends of the bell mouths for the time being as I don't have vacuum take offs in all intake runners to attach a vacuum guage (future project to drill and tap take offs for exactly this)

Here is a short course on carb theory, these are not from a book:

I'm still digesting all this, but after the third read it's starting to make sense. Thanks so much for taking the time to type it all up.

A couple of questions;
-What level do you set the fuel at using a dip stick down the main well and where are you measuring from on the carb?
-What's an AFR meter and is this the type of thing that your average enthusiast can afford?

Bigger chokes and main jets are bragging rights, and nothing else

I've got an Alpine, what more bragging rights do I need.

Once again, thanks very much for you replies. I'll now start the search for a supply of secondhand parts - new prices here start at A$18 each for an idle jet - I just can't afford a box full of new spare parts.

I'll get back to you all with progress over the coming months.

Regards

Graham

 

[RootesRacer]

No you dont need 45s, the point was that 36s in a 40 body doesnt leave room for a real choke profile. Anyone who put such a choke in a 40 did so on a dare. Weber probably only make a handfull, or someone turned down a 45 choke to fit a 40.


I dont recall the exact specs on the fuel level, but it had to do with a particular level in the main wells relative to the fuel inlet. It is the definitive bottom line with respect to TRUE fuel level.

I dont do carbs much anymore.

An AFR meter is an air fuel ratio meter.
For gross carb jetting could be a narrow band O2 meter, for finer jetting you need a wideband 02 meter.
Either require welding a bung to the exhaust collector.
Its a great investment, dont discount this advise.

As for the jets, look on ebay in the US or UK/europe. There are a few guys in europe that sell new jets for IIRC $2.50 USD each. Shipping was pretty cheap and they seem to ship anywhere.

Same with chokes.

 

[alpine_64]

graham,

is this car a white alpine with alloys and lowered.. i remember seeing pics of one in WA at some street race event.. had a large "ALPINE" stereo sticker across the top of the screen.. is this the car you bought?

As for Holbay heads.. they do not have the sparking plug tubes.. they are like the later alpine/hunter heads and have the caverns that the plugs go into. they also have a different shaped combustion chamber (and larger) and have the smaller intakes. If you have a warnerford manifold the holbay head would have had the intakes opened lots to match it.

There are a few places in melboutrne i can refer you to, but i think your jets and settings are probably VERY MUCH on the heavy side. Also.,. 95? try running V power or mobil syn2000.. they are 98RON.

if you want to chat i am happy to help give me a mailoff the board and ill send my phone number... also i would listen to Jarrid.. he knows what he's doing. Everything he said to me way back matched what the guy who set my werbers up did.. and hes a master at the black art of tunnign these things.. i dare say he might be able to help you over the phone.. or get you in the ball park.

 

[TheBrownHornet]

Hi Jarrid,

Once again thanks for the reply.

Anyone who put such a choke in a 40 did so on a dare

I like that. Your summation probably fits very well with the Australian psych and may be closer to the mark than you could imagine. End of the day, I have no idea, they were fitted when I bought the car.

I'll keep looking for the correct fuel level settings as at the moment I've just gone with the middle of the road 8mm set with the top cover off, but as you've suggested that doesn't take into account the angle that the carbs are mounted on. If anyone else can add anything here that would be great.

An AFR meter is an air fuel ratio meter.
For gross carb jetting could be a narrow band O2 meter, for finer jetting you need a wideband 02 meter.
Either require welding a bung to the exhaust collector.
Its a great investment, dont discount this advise.

Please forgive my ignorance, but is an AFR and an O2 meter contained in the same device or are they seperate instruments all together? I'm really interested in looking into this further. I have a family full of old car nuts who can and do share all sorts of tools etc around so an investment in such diagnostic tools can be offset by being put to use on a number of vehicles.

I'll have a lok through Ebay as you suggest for cheaper parts.

is this car a white alpine with alloys and lowered.. i remember seeing pics of one in WA at some street race event.. had a large "ALPINE" stereo sticker across the top of the screen.. is this the car you bought?

Hi Michael

This one's red, but I'm doing a little panel work to fix the right front fender at the moment and it has previously been white, and before that, BR Green. Chased up the guy who painted it red whilst trying to colour match. He remembered doing the car in 2002/2003 so it may be the one you saw pictured. I know the previous owner campaigned it in quite a few street events.

It most certainly has alloys. One set of 13" with semi slicks for racing and a set of 14" for the street. With the 13" fitted you'd be luck to drive over an ant without hitting it on the X brace.

It also had the huge Alpine stereo sticker across the windscreen. I thought it looked a bit tossy so it has been removed.

As for Holbay heads.. they do not have the sparking plug tubes.. they are like the later alpine/hunter heads and have the caverns that the plugs go into.

Sorry, but not sure what you mean by the difference between the sparking plug tube and caverns but if your able to give me any more detail it would save me pulling off the rocker cover and looking for casting marks.

If you have a warnerford manifold the holbay head would have had the intakes opened lots to match it.

There is an enourmous aluminium spacer plate about 25 (1") - 30mm (1 1/4") thick between the head and the intake and exhaust manifolds. I've been scratching my head as to it's purpose, but now that you mention it, it may be an adaptor to match between manifold and head ports.

There are a few places in melboutrne i can refer you to, but i think your jets and settings are probably VERY MUCH on the heavy side.

Any leads will be greatly appreciated. Having finally digested Jarrid's previous post I think I'm beginning to understand what is going on, and you guys are definitely right.
Larger choke leads to reduced air speed which required larger jets and bleeds just to get the fuel through. This probably explains why my idle screw needles were set at 3 1/2 turns as opposed to the recommended 1 1/2 - 2 turns recommended in the Weber book and elsewhere.

Also.,. 95? try running V power or mobil syn2000.. they are 98RON.

Will give it a try, but I'm not having any trouble with pinking at the moment. Possibly because things are so rich that it's simply not possible. Maybe octane will become more important once i get things set up properly.

if you want to chat i am happy to help give me a mailoff the board and ill send my phone number... also i would listen to Jarrid.. he knows what he's doing. Everything he said to me way back matched what the guy who set my werbers up did.. and hes a master at the black art of tunnign these things.. i dare say he might be able to help you over the phone.. or get you in the ball park.

I'm out bush for work at the moment, but thanks for the offer, I'll take you up on that in the not to distant future.

As much as my feeble brain will allow, I'm paying very close attention to not only Jarrid's, but everyones input. I really appreciate the willingness of everyone here to share their knowledge and experiences. The people on this site are truly great ambassadors for Alpines.

Thanks again, I have no doubt that with such a talented support crew behind me I'll have the Webers running as the factory intended in no time.

Regards

Graham

 

[alpine_64]

graham,

sounds liek the car.. photos i saw had it will 4 spoke escort/cosmic style RS wheels. The alpine sticker.. and low stance.. will be the same car.. am sure its it.. i might have got the colour wrong.

As for the head.. sparking plug tubes are round holes into the head that the plug vanishes into.. they make the head look nice and do a good job of seeping oil all over the place. The later heads have a cave shaped reliefe in the casting and the plugs are there.

feel free to give me a call when you get back.

 

[alpine_64]

Graham,

Seek and ye shal find..

this is your car?

 

[serIIalpine]

Interesting explanation of what the different emmulsion tubes do int e DCOE Carb here:

http://www.fastroadcars.co.uk/shop/index.php?act=viewProd&productId=85


Good prices even with the exchange rate and fast cheap service to the US, I might add.

Eric

'62 SerII

 

[Jeff Scoville]

I wish i could find some folks with as much knowledge about Dellorto's.
Maybe I should dump them and get a set of Webers.
To be honest though it doesn't seem to be too far off, and I found a shop that gaurantees to make it run perfect, at $110.oo per-man hour that is!

 

[RootesRacer]

There is no difference in DCOEs and DHLAs as far as tuning theory goes.

All the same rules apply.

The more compelling reason to use the weber is parts availability.

 

[TheBrownHornet]

Greetings Again,

Apologies for the tardy response.

Michael,

That picture is indeed my beloved Alpine. I think I even have a copy of the magazine with that picture in it. The car came with a huge folder full of clippings etc. I'll see if I can master posting a few photos when I get back home.

Back to the question of the Holbay head, mine has the "cavern" style spark plug holes as opposed to the tubes. At the very least that indicates a later style head.

Eric,

Thanks for the link. Those are the cheapest prices I've been able to find for parts.

Cheers

Graham

 

[serIIalpine]

I wish i could find some folks with as much knowledge about Dellorto's.
Maybe I should dump them and get a set of Webers.
To be honest though it doesn't seem to be too far off, and I found a shop that gaurantees to make it run perfect, at $110.oo per-man hour that is!

Fastroadcars.com are dealers of Dellorto parts as well. I have a friend with lotus Elite that runs Dellortos and gets everything from them.

They could probably recommend some jets etc for your Alpine.

Eric

'62 SerII

 

[Jeff Scoville]

Thanks for the tip. The guy that says he can tune them even said he can save me money by soldering the jets and drilling them to size rather then ordering a bunch of jets for trial and error. This seems to be the larger problem with getting someone to work on them is that they don't stcock parts for the Dellortos.
I guess that's what I get for trying to be different.
I'm working on trying to find an Alfa dealer/shop in the area as I'm told that is what they were stock on.
Once warmed up it idles beautifully, then at wide open it goes quite nicely, traffic/ stop and go in midrange is just a nightmare though, embarasing even.
I'm worried now that all the time I spent on tapping the mannifold and fabricating an equalizer tube has bitten me in the ass. Although I've checked and double checked I can find no vacuum leaks, I'm thinking of pulling it all off just to see what happens.
Oh well, if worse comes to worse I have four, two barrel downdraft manifolds which I'm sure I can find something to go on. I just love the looks of those sidedrafts though.

 

[RootesRacer]

Hey guys, I hope it would go without saying but you NEVER want to leave a set of soldered jets in a carb. Unless the solder alloy is identical to the jet, you get galvanic action that tends to grow deposits in the orifice. Also jet orifices have a profile, they are not a straight hole.

Ive soldered jets, but just in a pinch to figure out what range of jets I should order, then I get 3 sets, one up one down and one at the dimension I drilled.