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Can twin 40 dcoe carbs be tuned for economy
steve m - 19/7/11 at 10:16 PM

And yes i know the first answer

"drive slower/less throttle "

I had for a couple of years twin 40's on my 1700 xflow, and really liked evrything from the noise to the power, but did not like the fuel consumption, 10-15 mpg on a good day,

I am currently running a 1660 xflow, that is mildly tuned and would like the 40's back on the car, for asthetic reasons, but dont really want to go down to the unrealistic fuel figures, so can they be downtuned to allow this?

Steve

Paul TigerB6 - 19/7/11 at 10:27 PM

For economy, you may be better looking towards 3D ignition mapping rather than trying to lean out the fueling

jollygreengiant - 19/7/11 at 10:49 PM

I used to be able to get 30mpg out of a 1662cc full race tuned Xflow in a Mk1 cortina with a 3.7diff. Ok it had 11:1CR but, the trick was, and I have said it repeatedly before, to get economy, drive the vehicle with out moving the throttle. Each time you move (press even a little bit) the throttle you get 4 (YES FOUR) squirts of fuel into the engine (one per barrel), so control the vehicle speed by using the gear box, keep your foot still and don't get over enthusiastic about the glorious noise they make, cos you only get that throaty roar when you are using fuel.


edit bit. Oh and I had a top end of a smidgeon under 140mph reving to 8000rmp in top.

[Edited on 19/7/11 by jollygreengiant]

steve m - 19/7/11 at 11:22 PM

So thinking along your lines, could the acelerator pumps be adjusted to only come in at half throttle?
so in theory, up to half throttle, it is purely drawing the fuel thru the carb by suction ?

Steve

[Edited on 20/7/11 by steve m]

atspeed racing - 20/7/11 at 07:17 AM

The answer is yes.
Poor fuel consumption is down to poor carb set up or bad driving.
If the car needs 1 litre of fuel per hour to drive at 40MPH, then it would not matter how many carbs you had on the car, it would still need the same amount of fuel.

thefreak - 20/7/11 at 08:04 AM

Do you have a set of carbs ready or will you be looking around? Reason I ask is I've got a pair of 40s in the garage collecting dust

cliftyhanger - 20/7/11 at 08:18 AM

I know the later dellorto's were better developed than weber stuff, but on those I regularly got late 30's on a run, and IF i stayed legal may have broken 40mpg.

So yes, the carbs are poorly set up. I think some rolling roads just set them up for max power at full throttle, something that is daft on anything but a race car. In fact, if the operator is adequate, he can set the jetting to be very consistent across the rev range, tweek the pumps and so on. And all at little, if any, loss in power.

atspeed racing - 20/7/11 at 08:29 AM

with out getting too picky, a well set up carb will give the best power and the best fuel economy. The cam and big valves are the major culprit, meaning that a compromise has to be met.
Small valves and low duration cams will always help fuel economy, something that most twin carb cars do not have, hence the poor fuel consumption

MikeRJ - 20/7/11 at 08:42 AM

quote:
Originally posted by Paul TigerB6
For economy, you may be better looking towards 3D ignition mapping rather than trying to lean out the fueling


Exactly! Apart from poorly set up DCOEs (very, very common), the loss of the vacuum advance on typical DCOE installations is a big factor in the poor mpg which is often returned.

quote:
Originally posted by steve m
So thinking along your lines, could the acelerator pumps be adjusted to only come in at half throttle?
so in theory, up to half throttle, it is purely drawing the fuel thru the carb by suction ?



As long as you don't mind poor driveability in the first half of the throttle movement - the accelerator pump is not there no make more power, it's to prevent the mixture going weak when you open the throttle. Pretty much all non-CV carbs will have an accelerator pump of some description.



[Edited on 20/7/11 by MikeRJ]

Neville Jones - 20/7/11 at 10:38 AM

Steve,

Take the car down to Tom Airey, near Alton.

http://aireytuning.com/

Tom's old school, meticulous, and THE absolute best at getting the best out of Weber and SU carbs. He'll do a full carb and distributor setup in a couple of hours, and you'll never be happier for taking it to him.

Cheers,
Nev.

As you can gather, I'm a very happy ongoing customer!

steve m - 20/7/11 at 12:05 PM

Thanks for the replies,
I do have a set of twin 40's already, that im in two minds to either stick back on the car or sell, and buy some bike carbs

Having them set up buy a profesional is the answer, and apart from Nevs link, i have not found anyone near me

Infact the guy at Autec in crawley looked at me with a blank expression and said, "if it aint got an ecu, what the f*** are going to tune"

that inspired conidence, mind you he looked about 12

Steve

Craigorypeck - 21/7/11 at 11:26 PM

Agree with cliftyhanger! Later emission spec dellortos from DHLA F onwards are the best road car side draughts, you can adjust the pump jet diaphragm springs to meter the amount of fuel and the rate its delivered, the pump jets can also be down squirt which atomises fuel better, they also have emulsion tubes with loads of holes which also atomise fuel a lot more too, anyhow the best way to tune any carbs are living with them and tuning with an AFR meter hooked up to the exhaust manifold. If you like fettling that is!!!
My tuned 2.1 pinto was giving 35+mpg but it had megajolt on there too which helped things a lot...
What size chokes you got in there?? You can always choke it down at the expense of loosing top end.