Weber dcoe idle circuit mixture

[Mrmci]

Hi chaps,

In setting my carbs up I've had a query about the way the idle mixture impacts the mixture across the operation of the carburettor.

Does the idle control circuit set a kind of base level, which is then added to by the main jets. Therefore if you have a rich idle you're potentially rich across the whole operating window.

Or

Is the idle control circuit effectively redundant as the butterfly opens. I.e. it's only effectively influencing the mixture at idle and very low throttle openings and therefore the mixture across the rest of the operating window is only only influenced by the main jets.

Cheers

Mark
.

[nmauduit]

the ideal operation of the overlaps and transitions between circuits is usually depicted as below (your latter description)

weber circuits overlap

https://240260280.com/Tech/Carbs/Weber/images/Circuit%20Overlap.gif

this mainly results from the differing air flows at different locations according to the butterfly opening level, creating different depression levels at the fuel admission locations (e.g. little opening = strong laminar air flow at the transition hole right below the butterfly)

[DamianB]

Hi Mark,
Warning: I am no expert, but have spent some time recently trying to learn how my Webers work. I am posting here partly to provoke much more learned members to tell me how my advice below is wrong, and learn from them myself...

The idle jets are commonly felt to be misnamed. They influence your motor at RPMs a long way above idle. A great way to confirm this to yourself is to unscrew the jet cover off your Webers, remove the 4 x main jets, and take a drive on a quiet private track. If you accelerate gently you will discover the revs at which the main jets start to work (or more accurately, not work... engine will refuse to rev further). On my Elan it is between 2,000-2,500 RPM. So if most of your driving is gentle, you will be on the idle jets alone much of the time.

The best way to diagnose whether your idle jets are too rich or too lean is of course on a rolling road with an expert. Which is where I started. But to really learn about your carbs I recommend fitting an AFR sensor into your exhaust. I fitted one of these https://www.demon-tweeks.com/aem-electronics-x-series-wideband-uego-air-fuel-sensor-controller-gauge-aem30-0300/. That way you can fiddle around with different idle jets to find the right ones for your motor. The trick is to pick a Fuel jet size (the 45/50/55 number in table below) that gives you the right mixture at idle revs with all 4 idle screws set about 1.5 turns out. Then change the Air jet size (the F number in the table) to get the mixture right all the way up to till the Main jets kick in.

Good luck, and I hope you enjoy learning this stuff as much as I have

I found this table useful also, to help demystify the weber jet naming.

[alan.barker]

Hi Damian,
Many thanks for the info. I know it's not a Lotus but i fitted an AFR Sensor in the Exhaust manifold of my 1978 TVR 3000S.
someone before had tunned the Engine and never changed Carb Jets to suit.
The biggest change was the Idle Jets or i call them Transition Jets.
So easy driving along and watching the AFR change in different conditions
Alan

[mbell]

Both circuit work using vacuum, the "idle" circuit uses the engine direct vacuum caused by the throttle plate. So it works at small throttle openings but drops off as the throttle opens reducing the vacuum level around the idle and transition openings.

The main jets use a Venturi to generate a vacuum from the air flow through the carb body. So they pick up when there is a good amount of airflow, hence require the throttle reasonably far open.

So you can see these are designed to work together. The tricky bit it's the transition phase between the two, as idle circuit drops out and main circuit kicks. Very easy for it to go lean or rich and this is real hard but to tune.

The fuel size in the jet is used to limit max fuel provided. The air hole effects when/how that fuel is delivered. Generally a larger hole allows more air to be sucked through with the fuel weakening the mixture, but it means lower vacuum level is needed to draw fuel. So larger air holes lean the jet out but extend the operating range of the jet.

This can be used to tune the transition, a larger idle jet with larger airflow can help with a lean transition. Can also change main jets, air correctors, fuel levels, etc etc to change it.

[alanr]

DamianB wrote:-
'The trick is to pick a Fuel jet size (the 45/50/55 number in table below) that gives you the right mixture at idle revs with all 4 idle screws set about 1.5 turns out'.

Aiming for 1.5 turns out is only a useful approximate guide if you are tuning 151's or later Webers. If you have older 18's or 31's fittted to your car on which the adjustment screw is not so fine you need to be aiming for approximately 3/4 (0.75) of a turn out.
This is completely dependant of course on which jetting you are fitting or trying but useful as a given starting point for the idle mixture screw.

Alan.