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MG Midget and Sprite Technical - Weber twin 40's book recomendation

Hi All, My 1275 A+ engined midget has a weber twin choke 40 on it installed by the previous owner.

Given the previous owner was not technically minded and also how far out the mixture was plus it doesn't run that great I suspect it was just lifted off some old ford and bolted straight in.

I am afraid I know very little about Weber carbs but suspect I would benifit from swapping out some of the internals such as choke tubes, jets etc etc

What is the best book to buy to get an idea ?

Vizards A series book ?
How to power tune the A series ?

I may well swap out the cam at some point too and may run to a RR tune after that.

cheers

Ben
Ben Brown

Naturally I am biased :)

I think the best resource is the MGCC Midget register website here
http://is.gd/fXDs0

If you use the formulas here then you carb ought to be very close to perfect (consider it the same as a 45, the only difference between a 40 and a 45 is the size of choke that can be fitted)

After that perhaps a rolling road would fine tune it if required and if the operator had the parts and availability

GL
Robert (Bob) Midget Turbo

OK so now I am really confused

I would swear I have a 45 DCOE fitted only it has 40 DCOE2 No 27519 written on the top

Bob thanks for your link I printed it off and stripped my carb a little to see what is inside

Couldn't see any markings on the Venturi only a number 45. Measured the internal diameter at its narrowest point and I get 37mm to big for a 40 DCOE according to your link. According to the link I think I need 27 or 28mm so way out

The internals have the following printed on them -

Main jet 145
Air correction 175
Emulsion tube F2
Idle jet 45 F9

Looks like my 40 DCOE2 has 45 DCOE bits inside ?

Looking at your link I would say my carb came off a 2000 Pinto and was bolted straight on :-(

I think I need

Venturi 27 or 28mm
Main jet 108 to 112
Air correction 158 to 162
Emulsion tube F11 or F15
Idle jet F9 (Horray I have this one :-)

Help please .......




Ben Brown

Maybe it's just that somewhere in the past the top got changed over?
Alex G Matla

<< What is the best book to buy to get an idea ? >>

If you're fond of mind-numbing detail, John Passini's "Weber Carburettors: tuning tips and techniques" is a fine fireside read. Not DCOE specific, but plenty about them, as well as good general information on the important as well as the arcane.
Richard Reeves

A 40DCOE2 is off a early alfa or lancia, we have them on a elan (fitted by the previous owner). apart from subtle differences (things like the idle mixture volume screw seems to be much courser in its operation) there may be other parts which differ such as the throttle plates angle (stamped on butterfly) and progression drillings.

also being virtually the oldest version, and fitted to a 'every day' car, they may be quite worn by now check throttle spindle bearings.

i have a very good book done by the same publisher who does mssr Stapleton's book hers a link to the most recent edition on ebay.

http://cgi.ebay.co.uk/Build-Power-Tune-Weber-Dellorto-Carburettors-V275-/200527664883?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2eb06156f3

danny
d cusworth

If the venturi has 45 on it then it's not the main venturi and is therefore not definitive in establishing if your car is a 40 or a 45. The part with 45 on it is most likely 4.5 and the auxillary venturi.

Look at the photo attached - what does the 4.5 part you removed like? The middle part or the part directly behind the horn/ram/trumpet that has been removed?


Daniel Thirteen-Twelve

Hi Ben you have understood the chart exactly although I like to think that an engine equiped with a weber ought to be revved ever now and then:)

With that in mind I would always at least fit a 30mm choke or more often a 32mm

Parts for webers are not that expensive so why not start with the 30mm and see how it goes.

so should be

choke 30
main jet 120
air corrector 170
emuls tube F15
idle jet 45 F9

When you order you could buy a set that matches up for 32mm and hopefully you will be sound. If you do buy the 32 mm set as well buy the idle 50 F9 I have always found I need more fuel at idle/slow running. It must be remembered that when the engine is cruising this jet is fuelling the engine up to about 60 MPH so it needs to be right. So nice to have a choice to get the right one.

Good Luck
Robert (Bob) Midget Turbo

Hi Danny, the part I removed was directly behind the ram pipe.

I just took off the air filter, unbolted and removed the ram pipe and just pulled it out. It looks like the part in the picture that is between the two ram pipes i.e. if you drew a line on the diagonal between the two ram pipes its that bit in the middle.

cheers

Ben
Ben Brown

If this is the biggest bit in the picture and not directly behind the ram that is removed than your car's carb is a 40DCOE. Stand it on one end (like a chimney) and measure it. If it stands more like 60mm high than 30mm, it's definitely the Aux Venturi from a 40DCOE (not the 45 DCOE). I suppose it might be as quick to measure the throttle butterfly.
Daniel Thirteen-Twelve

Just found this picture

http://www.carburetion.com/diags/40DCOEDiaginfo.asp

The thing I pulled out 17 is the auxilary venturi

Will the main venturi just pull out too ? the same way as the auxillary one ?

cheers

Ben
Ben Brown

Just answered my own question yes it does pull out and it was a 36mm choke so as big as possible.

Where is teh cheapest place to get all these bits I need ?

eBay ? - Cant see any on there at the minute

cheers Ben
Ben Brown

Bob, very nice and understandable description on the weber selection.
But what about the auxiliary venturi size?
There is no mention about it?
Alex G Matla

Thanks Alex

In the DCOEs I have used the Auxiliary venturi is simply the insert in the carb throat that injects the fuel into the air stream and it is not sized but simply fits what ever size dcoe it is fitted into.
You do not need to worry about it. :)
Robert (Bob) Midget Turbo

On a 40DCOE there are only 2 sizes of auxillary venturi and Ben's carb has the 4.5.

On a 45DCOE there is a wider choice of sizes.

However, the DCOE on an A-series engine doesn't seem that fussy about what size is used. What you do need to watch is if copying jetting figures from a book you also go with the aux vent they are relevant for. Also you cannot swap aux vent sizes in isolation you need to swap main jet sizes at the same time (assuming the calibration was spot on at that time).
Daniel Thirteen-Twelve

Well, I've got 3,5 aux venturi's in my 45, any good by chance?
Alex G Matla

If the rules do not work come and shoot me.
LOL.
Seriously I do like feed back from people who try setting up webers to see initially how the engine runs and secondly if they go to a rolling road how much it is reset?
Bob Turbo Midget England

Bob, I'll be using the weber soon again, after I've rejetted it accordingly. Right now I run double HS4's and got 82,5 HP @ 5000 rpm at MGlive RR (calculated flywheel), was still running in. At the moment it feels much more powerfull, so I think the engine "came loose". I don't think I'll be on a RR soon, maybe at midget50.
Alex G Matla

I used 3.5 Aux vents - with the jetting calibrated on the rolling road for max power. I swapped to 5.0 Aux vents at the same rolling road session, recalibrated for max power (just main jets I recall) and the power was the same. The power was also the same with a K&N air filter fitted and without.

It's possible a more powerful engine might wring out a bit more power on the 5.0 Auxs or that the engine might be a bit more economical (better fuel atomisation??) with them, I don't know.
Daniel Thirteen-Twelve

This thread was discussed between 11/10/2010 and 15/10/2010

MG Midget and Sprite Technical index

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