Weber chokes question

[Gridgway]

Further to my nuts query (which Roger has rather deftly answered), I am confused by chokes.

In the diagram here there are parts #17 and #22 which are "Auxiliary Venturi" and "Venturi (Choke Tube)" respectively.  My engine builder, in his words "put bigger chokes in" and gave me back the ones he took out in the box of leftover parts.

So what he put in were #22 and what I have spare is part #17.  There's no way they'll both go in and they are different constructions.  #17 has a centre "jet".

I keep thinking it should be obvious! Can anyone shed any light on this?

[jbcollier]

Not sure what your question is.  The choke is a restriction that creates the pressure differential to draw fuel in.  The auxiliary venturi is the outlet for the case's main circuit.  Both work together, along with other parts, to control when, and how much, fuel flows into the engine.

Smaller chokes work better with low air flows and larger chokes the reverse.  Choke selection is based on expected engine air flow and is a compromise either emphasizing low end torque or high speed power -- or something in the middle.

[mikestifel]

The chokes and auxiliary venturis will be numbered.  It's pretty easy to see the numbers on the chokes, so you should be able to tell what you had and what was installed.

[OldAndrewE]

They do both go in, in the order shown on the diagram.  It is part 22 that will be changed and they are available in something like 24 mm up to 36 mm for a DCOE40 carb.  If he changed 22s then you should have a spare set of them, if not then he has kept them. Were you charged for new chokes, some will just swap and keep the old without charging as they will all be useful somewhere.  At around £20 each they are worth having

[Roger King]

Yes, as AncientAndrewE says, they both go in and are both required for the carb to function. Part 22 is the choke (or venturi) and part 17 is the aux vent (auxiliary venturi). The choke's job is to accelerate the air as it passes through the carb and thus lower its pressure, and the aux vent does the same thing, but in a localised area to lower the pressure even more in order to suck fuel through main jet and into the air stream through the annular centre of this part. It is more usual to change the choke size than the aux vent.

So check that you have one of each type of part in every barrel and that they are all the same size. The chokes have their internal diameter cast into them though you may need a torch and/or mirror to read it in situ. It will be somewhere between 30mm and 36mm for 40DCOEs and a little bigger for 45DCOEs.

Now, I think there are two possibilities here. 1) You need to learn more about carbs (not meant as an insult, I was the same once).  2) Your carb guy doesn't know what he is doing (this may be grossly unfair without knowing more).

Without knowing more about your engine spec and installation it is impossible to make further useful comment, but a Supersprint will run 32mm or 33mm chokes in standard form.

[OldAndrewE]

Graham,

This is a useful book to help you learn about these carbs How To Build & Power Tune Weber & Dellorto DCOE, DCO/SP & DHLA Carburettors 

It is worth making a note of the main settings for your carbs as they are now (and as Roger says make sure they are the same for each barrel)

Perhaps Roger will comment but I would think is unusual to change choke size without changing anything other settings

[Gridgway]

Thanks Roger and firstly you are absolutely right that I need to learn more about carbs and feel rather foolish!

I just went and properly looked with a torch and lo and behold the truth became clear.  The bigger chokes were still in the carbs of course.  They are 36mm and the ones that were previously in there when the engine was a standard 1600 were 30mm.

It's now a 1700 supersprint (and a bit).  The backstory is that the (upgraded) engine was not happy and running well enough for me to run it in.  So it went to a local to me RR (not the guy who built it) just to get an initial setup that was reasonable for me to drive around.  The verdict was that the chokes were too big to get a setup that worked.  So I took it home to see what I had got.

So it seems to me that the 30mm chokes will be too small and if the 36 are too big then I probably need 33mm chokes.

So I have learned a bit more about webers!

Thanks again.

[Roger King]

Don't feel foolish - you are learning "on the job". 36mm is far too big for a Supersprint, and is indeed unusual to use in 40DCOEs, being more common in 45DCOEs. Oversize chokes do nothing to increase power, but will lose you a considerable amount of low to mid-range drivability.

If you change the chokes you will always have to change the jetting, assuming it was initially correct. Basically, you select a choke size suited to the engine capacity and specification, then set up the jetting to suit. This can range from simply changing the main and air jets to also including the emulsion tubes, slow run jets, pump jets, etc. A competent rolling road operator is needed for this unless everything is to an exact known spec in which case you may be able just to change the jets to a setup known to suit the engine.

So far as I know (out of the business now for twenty years), 33mm chokes are not available, so you may need to settle for 32mm or machine out some smaller ones. If you do this, make sure that whoever does it maintains the correct internal profile of the choke or you'll be worse off than if you stick with 32mm.

[Gridgway]

Thanks Roger.  The engine builder really wanted us to have 45DCOEs, but the old engine came with a set of 40s that were refurbed, so decided not to invest in new carbs at the moment.

Now very strangely, the one choke I stuck my fingers into came out easily.  But the other three refused.  Having a google revealed (from blatchat 2001) that to change the chokes there's a screw and lock nut to remove as well as the idle air bypass screws (that was your comment Roger!).  That begs a huge question about how the one choke came out.  I will have to inspect to see if I can work out what's going on.  Feels like more than anticipated!

The rolling road guy is a long termer in this Ford/Caterham world with lots of xflow and BD build experience so I'm happy he can make it all work.  The long term likely outcome is to change to 45s, but if I can get the 40s working with a drivable rev range not too lean nor rich, we'll be happy to drive round until the spring.

Webcon website seems to allow you to add 33mm chokes to the shopping cart so maybe they are available (made in China I assume).

[Gridgway]

A quick update.  In consultation with the RR man, his recommendation was to go for 32mm chokes which I ordered and they arrived today.  I could only get one of the chokes out by hand, so I bit the bullet and took the carbs off to be able to tap the chokes out which I did.  

I fitted the new chokes and in doing so found one the aux venturis wouldn't go in as far as the others by 1-2mm.  After some amount of taking out, looking, head scratching, I compared it with the others and its overall length is greater than the other 3 by about 1.5mm.  The normal perfectionist in me wants 4 identical ones, but if I order a replacement, I don't know whether it will be any better.  So I have put it all together and the engine starts and runs well enough for it to go on the trailer and off at the other end for the next RR session.

I also had some fun with the trumpet/backing plate studs, but I'll update the other thread.

And Andrew thanks for the reference suggestion, most kind.

[OldAndrewE]

Do the aux venturis have any part numbers on them?  If so I guess you have checked they are all the same

Might be worth a call to the supplier, I would think they could advise on the length issue

[Gridgway]

No part numbers on them Andrew and it's what was in the carbs from when I bought them so no supplier as such.  They are very early carbs (can't remember the manuf date).  The new ones on Dellorto are £33+VAT and also the later type held in with a grub screw.

[OldAndrewE]

OK Graham, when I said supplier I meant the supplier of any potential new one and explain your concern about different lengths.  Like you I would prefer to see all four barrels identical

[Gridgway]

Yes, it feels like the way forward is to buy 4 new ones.  I think the ones that Dellorto have will work, they just have an indent for the (modern) grub screw.  It's just a question of whether it's worth buying 4 to get them all the same.

Also I think that the engine needs 45s (according to the builder) so I should perhaps just focus on making the setup I have ok then bite the bullet for the 45s.

[mikestifel]

What are your goals for the car?  There's considerable overlap in performance between 40s and 45s depending on chokes etc.  If you're on the track and need absolute WOT performance the 45s might be the ticket.  If it's a road car you can probably get very good performance out of 40s.  Just be careful and think about what you want from your car.  DCOEs are great when they're set up correctly but the cost of jets and other parts can add up in a big hurry.  Finding someone who knows what they're doing can make a world of difference.

[Gridgway]

That's a really good point Mike and it's why I am getting the car set up in its current form for me to use.  It's going to be predominantly road use, so drivability at all throttle openings is key.