I plan to connect AFR sensors to my SU carburettors using an arduino, driving an air valve to control the jet needle.
SU carburettors use a piston in a vacuum chamber to lift the needle of the jet assembly in order to regulate the fuel flow mixed into the air stream, assuring (or trying to) a constant mixture richness.
The idea is to connect the vacuum chamber of the carburettor to the intake manifold via an external adjustable valve allowing the arduino to assume the fuel regulation .
So I am actually looking for a 12V proportional air valve, and that is the subject of this topic.
So how does your design measure the mixture richness? As I recall, my 1959 Sprite had SU carbs with tapered needles so the mixture could be richer under acceleration and leaner under idle conditions.
I am not an engineer and not yet read much litterature about richness ratio under different engine acceleration rates.
On my SU HS8, the reachness under acceleration is modified due to the oil dashpot preventing the piston lifting immediately, so the surface of the intake stays small enought to enhance the ventury suction effect, allowing the carb to swallow more fuel at this instant.
Concerning the air valve, its diameter should be under 1/4" and it could be driven by an external servomotor.
You will need drive circuits and linearisation for your oxygen sensor .
Can’t see this working as the adjustment needed with vary with the needle position , so you need to know the needle lift and opening will change rapidly .
The vacuum connections are internal on the SU carb , so you will need to modify this and take care of transient enrichment
Also if you need more vacuum than is available your adjust can only work in one direction.
Get the carb serviced and the correct needle installed !!
It’s a complex task and you would be better going to fuel injection, there are diy systems about
Thank you for your contribution Hammy. You are absolutely right on all these points, and I would like to discuss them with you but I fear that a forum dedicated to arduino is not suitable for a conversation concerning the adjustment of SU carburetors. However, I would like to explain the reason for my approach. When trying to adjust my carburetors I noticed that while driving the richness fluctuated constantly and very rarely stayed around 14. I then wondered if controlling the richness via an arduino would be a good thing, but on reflection it seems not. I had better to choose different needles among the 700 items of the SU catalog if needed ...
In any case, thank you all for your help.