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Jetting a 40 DCOE for 1275

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twas_brillig

Jedi Knight
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I've got a complete 40 mm DCOE (c/w 33 mm Choke (measured and casting label) and Auxiliary Venturis labelled '3' and '1', but both labelled either '4.5 or '45' and measuring out as 35.5/36.5 (my measurements, from each end); plus a 'junk' 40 DCOE with 30 mm chokez and 37/36.5 mm auxiliary venturis (my measurements) both of which are labelled 'S' and '45').

We'll be installing a 1275 cc engine n the BE with the only performance modification being an LCB header and larger diameter exhaust pipe and an H-D muffler (maybe) .

Questions:
Q1 - I've got some of the standard Weber references (Haynes; Passini Volume 1) and the consensus is to use a single 45 with the 1275 or a pair of 40s. However, I believe that the standard SU combination for a 1275 is a pair of 1.25 inch (31.75 mm) which suggests that a 40 mm DCOE (1 5/32 inch) should be adequate. And since I own the 40 and a goose-neck manifold for a single DCOE....
I would appreciate any advice regards jetting/chokes/etc. etc. for using a 40 DCOE on a 1274 cc engne

Q2: can anyone translate/advise as to identifying the auxiliary venturis? and what all those gosh-durned numbers mean?

thanks,

Doug
 
The 40 DCOE will run on the 1275. If you are looking for the full potential for your engine then you have to run the 45 DCOE. Jetting is not cheep to start with, so my best advice is get a 45 DCOE and dial that in.
 
Your BMC A series engine uses siamesed inlet ports so running a pair of DCOEs doesn't make sense. Remember that the DCOE is nothing more than two single barrel side carbs that share a common float bowl. If you were to run a pair of DCOEs, you now effectively have four carbs but only two inlet ports that you can feed. So given that, yes a single 40DCOE should work for a a 1275 A series engine, but there is a problem with your conversion calculation, you should revisit your numbers.

As for the aux venturi sizing, it has nothing to do with the larger diameter or the length. It is tied to the equivalent diameter of the fuel feed from the emulsion system of the carb. The smaller the number, the leaner the aux venturi, the larger the number, the richer the aux venturi. The numbers would be along the lines of 3.4, 4.0, 4.5, and such. Other than that the stampings are not clear or someone had added some marking, I cannot explain anything that you see that doesn't follow that format.

Something I must ask given the lack of modifications is why are you considering going the DCOE route? Both the beauty and dread of a DCOE is that it is infinitely tune able, it's very easy to go down a rabbit hole and not be able to find your way out. Seems to me that perhaps you should look into perhaps fitting HS4 (1 1/2") SU carbs in lieu of the standard HS2 (1 1/4") SU carbs that were fitted to 1275 Spridgets. Much more flexible and not nearly the potential tuning hassle of the DCOE.
 
I think Tybalt makes some good points. If you're not changing the cam and doing any head work, you may be able to get by with the 1 1/4" SUs with a richer needle. Messing about with the DCOE seems like a big hassle to me if you're not racing the car. If you want to improve your motor's performance, you can't do much better than any of David Vizard's books on tuning the A series motor. This is also a great video of one of his presentations:

 
Man, sure had a brain fart when I mentioned the dual 40s - I mindlessly picked that up from one of the reference books and they were probably talking about the 1500 engine. Regards why: well, I've got this carb....
Back in the 70s, I got incredibly frustrated with the SUs as the shafts were worn and I couldn't resolve that nor find someone to do the machining etc. to take care of hte problem, so bought a Weber 40 DCOE and manifold, all jetted for the 948, and bolted it on and never had a problem. Plus I like the looks. But I've got a pile of SUs and associated manifolds now so that recommendation (using the SUs) is probably the wisest. And if I really felt wealthy and adventurous: stick a couple of injector bosses onto the goose-neck manifold and convert to Megasquirt (I hear that it can now handle siamesed intakes, but don't know for sure) and use the DCOE as a throttle bottle, and have a 'stealth' EFI system that would still look carburetted. Thanks for the suggestions and knowledge. Doug
 
Twin DCOEs are possible on an A-series if you split them with an appropriate manifold. Bit more of the theory here: https://www.turbominis.co.uk/forums/index.php?p=vt&tid=454792 And they do look the business:

split-weber-dcoe-sprite-a-series.jpg
 
Too many, as in WAY too many, have tried to go the way you are using a DCOE (or two) on the 1275, and tried to get the jetting correct. The most common problem that they cannot solve is "off idle stumble". Most are trying to put carbs that are too big for the application. It's not that can't be done, but you'll need a complete of jets and the TIME to experiment. It may cost a little more to take it to an expert, and have them set it up on a dyno and let them custom tune it. You'll find it money well spent.. At least that's the way on consider it having done so with my Weber DGV.

Mine is a little cold blooded, but once it gets a little warm it idles just fine and the idle is the same no matter what the weather. That is When it's 45 degrees outside, the idle is the same as when it's 95 degrees outside and the car is at the end of a good run. (unlike SUs) I have no off idle stumble, and there is only a hint of hesitation if I'm doing 65 mph and suddenly floor it, (as the secondaries kick in) If i just press the pedal down that momentary hesitation doesn't show up. (It's kinda like kicking in the afterburners)
 
Good Grief! I'd probably hurt something (my brain) if I opened a hood and saw twin DCOE's amounting to two single barrel carbs. Thanks for pointing that out Rick, and the link for the discussion. I'm surprised there aren't four intake trumpets, just to really confuse us yokels looking under the hood. Doug
 
From SpriteSpot.com, in 2006:
******************************************************************************
40 DCOE on a 1275 A motor: Fuel mileage will be down 5% from two SU's but drivability should be good with the right setup.

32 choke
4.5 aux venturi
140 main
F2 emulsion
175 air
45F9 idle
50 pump

IF you need a rebuild kit and some brass you might spend a c note or even more depending on what you have to change. IF you got the HS2's with the 1275 you might be better off using those.
*****************************************************************************
There was no comment as to the source of the Weber recommendations; the HS2 recommendation can be attributed to common sense. Doug
 
From MiniMania: " even a stock 1275cc only needs 35mm chokes; all of these engines should use a 40DCOE". Sigh! Sure glad this forum doesn't get tangled up with economics or politics, as carbs are confusing enough. Doug
 
twas_brillig said:
From MiniMania: " even a stock 1275cc only needs 35mm chokes; all of these engines should use a 40DCOE". Sigh! Sure glad this forum doesn't get tangled up with economics or politics, as carbs are confusing enough. Doug
Click to expand...

That's kind of what I was thinking for a stock 1275. If you figure just choke diameter the 40 DCOE is a tad bigger than stock. Carb sizes can overlap quite a bit. I thought I would remember a chart from a while back that gave SU sizes for various HP outputs. That's what I used to pick a single HS 6 for my 1500 Datsun engined Midget....good for 80HP and that is all I expect. Could I get a couple more ponies out of a bigger carb? Yes, but I love only tuning one SU and the way it run's.

Kurt.
 
twas_brillig said:
From SpriteSpot.com, in 2006:
******************************************************************************
40 DCOE on a 1275 A motor: Fuel mileage will be down 5% from two SU's but drivability should be good with the right setup.

32 choke - same
4.5 aux venturi - 4.0 aux venturi
140 main - either a 140 or 145 main jet
F2 emulsion - F7 emulsion tube
175 air - 185 air correction jet
45F9 idle - same
50 pump - same

IF you need a rebuild kit and some brass you might spend a c note or even more depending on what you have to change. IF you got the HS2's with the 1275 you might be better off using those.
*****************************************************************************
There was no comment as to the source of the Weber recommendations; the HS2 recommendation can be attributed to common sense. Doug
Click to expand...

That's not too far off from my initial thoughts on setting up a 40 DCOE for street use on a 1275 in your previous thread asking about jetting, see post #5:

https://www.britishcarforum.com/bcf/showthread.php?104299-Weber-for-a-1275-cc-engine

I went with the 4.0 aux venturi due to your stated elevation since it would lean things about a bit for that thinner air. Went with the F7 emulsion tube for the same reason. The F2 emulsion tube has a thinner wall so it flows more fuel and is generally intended for richer set ups with big main jets on heavily modified engines, not so much intended for street use. Based upon your earlier post concerning no change in valve train or performing head work, modifications were all bolt on and limited to the LCB exhaust manifold and larger diameter exhaust plus the change in carb set up, the F2 strikes me as being over the top in your particular application.

The 185 air correction jet was selected based on the general relationship of main to air correction jet size for the DCO series. See my initial "guesstimate" set up in bold above from that earlier post compared to what you have listed from SpriteSpot. Then again, bear in mind that this was an initial set up "guesstimate." The final set up configuration should be based on testing and changing out calibrated components based upon the results of the testing to arrive at that final set up. On the bright side, you only have to have two of everything since it's a single DCOE set up. Imagine having to do all of this with a triple DCOE set up where you need six of everything, the jets aren't so bad, but it can run into some real money when you start swapping out the bigger bits of calibrated components like venturi and emulsion tubes.
 
Regards needing six of everything: when I win the lottery (a LARGE one), I'll hire a guy with an Italian accent to install triple Weber DCOEs on our AH 3000, and budget for having him live in the garage every summer!
More seriously: thanks to everyone for the suggestions. I just found the contact information for the shop in New York we bought a 40DCOE and Cannon gooseneck from back about 1980 for our 948 cc BE - they're still in business, but looks like they moved to rice rockets. Still dropped them a note in case they're still into Webers, and to say 'thanks' for a combination that's still working for our yellow BE.
I also ran across Redline and dropped them a note regards choking/jetting etc as they sell a 1275 kit using the 40DCOE, and asking if they would sell me the various bits.
And it turns out that in 2009 I asked this Forum the same gosh-danged question (we'd had the paint and body done about 2007) with the hope of getting the car together. I'll pull together the figures from the first 2009 thread plus this one plus whatever I hear from Redline (and Tybalt and Nomad et al) and put together a very long summary. Thanks everyone! Doug
 
From https://www.peterrenn.clara.net/midget.html#engine

[FONT=Arial,Helvetica][SIZE=-1]Weber 40 / 45 settings.[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]Although the Vintagent in me prefers the prehistoric SU, The Weber 45 is pretty much the standard carb used by racers. For roadgoing applications though a 40 is fine. See Vizard for full chapter and verse on these but the following settings are quoted by Moss in their Special Tuning Manual:

[/SIZE][/FONT]

[FONT=Arial,Helvetica][SIZE=-1]Road setting. Weber 40DCOE[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]30mm Choke[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]120 Main Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]45 Aux Vent[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]F16 Emulsion Tube[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]155 Air Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]35 Pump[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]40F8 Idler Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]This is what is fitted to Andrew's green 1275 car (see garage page) so we know it works. It pulls as well if not better from all speeds. Significantly better than the standard twin SUs, especially on low-speed pickup. Sounds good too!

[/SIZE][/FONT]

[FONT=Arial,Helvetica][SIZE=-1]Fast Road setting. Weber 40DCOE [/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]32mm Choke[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]145 Main Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]45 Aux Vent[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]F16 Emulsion Tube[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]180 Air Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]40/45 Pump[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]50F8 Idler Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]This I have no direct experience of, but I would guess loses some low revs tractability. You'll need a hotter cam and/or high lift rockers to exploit the difference, otherwise I doubt it will do any more than the Road setting except burn fuel.

[/SIZE][/FONT]

[FONT=Arial,Helvetica][SIZE=-1]Race setting Weber 45DCOE [/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]40mm Choke[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]190 Main Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]45 Aux Vent[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]F2 Emulsion Tube[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]170 Air Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]60 Pump[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]50F6 Idler Jet[/SIZE][/FONT]
[FONT=Arial,Helvetica][SIZE=-1]This is Moss recommendation and will, of course be a starting point as any race engine will have to be sorted out on a rolling road to optimise it for all the other variables in cam, ignition, exhaust etc.[/SIZE][/FONT]
 
MiniMania's web-site suggests that a 40 DCOE with 34 mm chokes is suitable for a stock 1275 engine. I dropped them a note regards jetting and received this very prompt reply:

"Perhaps this is a better starting point:

Choke : 35
Main Jet: 145
Air Corrector: 180
Emulsion Tube: F16
Pump Jet: 45
Idle Jet: 50F2
 
I have had Weber's in the past
on a cortina ,a celica and mini and a DGV on the bugeye
it seemed the we were always changing things with intake timing cam exhaust to make up for the shear amount of fuel they would dump.
on the cortina we were le to make it work AFTER spending buckets of money on head work exhaust and cam,
but it was a hill climb car and to try and drive it on the street was impossible.
i had a a tackle box full of jets and chokes Syc. tools gawd it was crazy.
life is to short to be spending it with a DCOE OR A DGV
Find a nice set of SU's call Joe Curto In NY. for a nice rebuild kit and enjoy the sound of singing SU'S
A nicely tuned SU out performs a Weber all the time why?
because there near I'm possible to tune for every day.
to finicky
rebuliding Su"s is like a ZIN thing for me . Make it sing!
weber is just pure frustration.

FYI
you mentioned the worn shafts - bushings
90% of the rebuilds I have done the bodies are fine and just replacing the shafts is all that's required so no maching - bushing pressing is required on the HS 2's
 
We've got the Yellow Sprite with the Weber that has given us yeoman service on a 948 plus I've got two Weber 40 DCOEs lying around with one goose-neck manifold. I've also got several sets of the 948 SU carbs, and may have some of the larger ones in a bin about an hour and a half away. I'm in touch with a chap who used to run a british automobile repair/resto/mod shop until he had a couple of strokes and retired about five years back; he was going to build himself a 1275 using All The Good Parts (or, at least, pretty good affordable ones) for his Bugeye but he might build/sell it to me, and I'll use it in the Silver Sprite instead of the 1275 sitting there. He still offers SU services and may have a set of HS2s (I'm actually hoping he might have some indestructible axles) in his old stock. I think he said he's built about 190 of these engines over the decades, and I expect that I'll go with his advice. Thanks for all the discussion and wisdom. If nothing else, we've got a swack of recommendations for anyone thinking of following the White Weber Wabbit down the Wabbit Hole. Doug
 
IT RUNS!
Tried starting the Silver Sprite 1275 a bit ago and it would fire but not run. I'd installed a DCOE 45 with the internals as recommended by Redline in an early 90s manual. It got pulled, disassembled and soaked with carb cleaner (which has an odour that is dang near impossible to get out of your garage or your clothing); reassembled and bolted it on and it fired up today, and idled, and revved! Need to do timing and dwell etc., but at least it starts.
The configuration we've got is:
Choke 34; Aux Venturi: 3.5; Main jet: 140; Emulsion tube: F2; Air: 175; Idle: 50F9; Pump Jet 50; Pump exhaust valve: 50. The Redline manuals recommendations nearly matched a 45 that Jeff picked up as a pair for a BMW 2002 so it was (relatively) inexpensive to follow their recommendations. The 1275 is stock with an LCB header. I'm sure folks could optimize this combination but I'm just incredibly tickled it fired up - just in time to put in winter storage.... Doug
 
Whoops! early 80s manual. Doug
 
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