SU tuning

[Geoff101]

I've now got my car (RP saloon) running with the SU carb and the fuel staying mostly on the inside of it, but I need some advice on tuning it.

It's a rebuilt engine. Standard valves and cam with the combined inlet / exhaust manifold. Standard exhaust.
It has a high compression head which was skimmed a fair bit to flatten it.
I'm using a DK4A distributor but not sure what advance profile it has (LC or HC).
Timing is set slightly advanced from TDC at idle. Should this be dead on TDC?

Carb is an SU H1 (1 1/8") with no spring and the brass weighted piston.
The carb was bartered from another owner. It's since had a complete new jet bearing assembly (the superdry version) as well as a service kit with new jet.
It currently has a VS  needle which looks from the charts to be too rich for a fairly standard engine.
Jet has been centred and the piston rised and falls smoothly by hand and drops with a click.
It doesn't fully open when you rev it with no load, but I've read that's not unusual on a standard engine.

At present, the car starts and runs pretty well but all indications are that it's running too rich, both at idle and on the move. I have a lambda boss welded into the exhaust with an AFR meter in a mobile box connected to it. 

At present, I'm struggling to get the mixture weak enough at idle and still have it run smoothly. Tightening the mixture nut (to make it leaner) makes the idle miss. A smooth idle that sees the highest tickover shows about 10.3 on the AFR meter (too rich). When running the mixture enriches further but that's possibly to be expected with that jet.
The above is born out by sooty plugs.

As I understand it, the selection of jet shouldn't make that much difference at idle unless this one is worn??

Any advice on where to start is appreciated.

[Malcolm Parker]

I would recommend using either an EB (Mini 850) or AN (Morris Minor 1000) needle, they are known to work well on a standard engine.
If your needle is too rich, try altering its position in the piston.  Move it out so the shoulder on the needle is about 1/32" proud of the piston.
If that improves things, all well and good!

[wild_willy]

The problem with using a AFR meter to set the idle mixture relies on sufficient gas flow past the oxygen sensor. Unfortunately this doesn't happen and will give you false readings. Adjust the jet so that when you lift the  piston with a small screwdriver the revs should increase slightly and then reduce as you further lift the piston. Sounds like you can set the idle mixture Ok when you say the engine starts to miss when you screw the jet nut all the way up.

I am running a 1" OM SU on a 3 bearing engine with a AH  needle (probably need to check that). Running modern unleaded fuels will need some additional advance. A DK4 should give about 16 degrees advance so maybe start with about 5-10 degree initial to give a total of around 20+.
Don't know about a VS needle but can look it up if you like.

[Stuart Giles]

Lifting the piston as wild willy describes will indicate whether your mixture is too rich at tickover. If the car is running significantly rich on the road and you don't have a high a fuel level in the jet, lightening the piston weight will weaken the running mixture. Needle changes are really about fine tuning once everything else is about right.

[Charles Levien]

I would recommend using either an EB (Mini 850) or AN (Morris Minor 1000) needle, they are known to work well on a standard engine.
If your needle is too rich, try altering its position in the piston.  Move it out so the shoulder on the needle is about 1/32" proud of the piston.
If that improves things, all well and good!

Im using an AN on a 1” and it gives good performance — excellent plug color, good pick up and low tick over.

[Geoff101]

Thanks. I've just fitted an AN needle and will try to set the idle mixture as above when I get a moment.
Cooking has taken priority now!

[Ian McGowan]

Malcolm's advice was, as usual correct. You are now in the ball park with the AN needle and should be able to adjust in the normal way for best running.

[Geoff101]

The AN needle has stopped it getting richer when accelerating. Setting the idle mix as above is still rich according to the AFR meter but perhaps I need to check that against something calibrated!
Rattles along well enough now though.

[jansens]

I have a good AFR meter and I have found it's easy to get too caught up on trying to get the reading 'correct' whatever that means. It's a handy tool but I think A7s seem to suit tuning by ear and feel better starting with a needle known to be about right. And you need to drive it of course to really know. Or do what I will probably do, ask a friend who knows to help (Ian, next time you're down!).

Having one tool to measure something can be annoying but as soon as you have several and they all give different readings then you get really confused. Trying to measure temperature accurately is a good example. I have countless meters and thermocouples and thermistors and IR contactless things and they all give different readings. Most accurate thing I have is a lab grade old school mercury thermometer!

The main reason I got my meter was for when I try supercharging the car and then I am in somewhat more unknown territory I and want to make sure I am not way off in the settings.

Since I can't go driving anywhere it isn't a priority just now and I am in the middle of remaking the exhaust at the moment anyway. But once that's done I will try the trick of putting the marked stick in the piston so you can read off what station the needle is at and make adjustments appropriately. That's described in the Des Hammill SU tuning book.

Simon

[Ian Williams]

In the OP there is one thing which is not clear to me, you say it has a new jet but you refer to a needle as a jet also. If the jet and for that matter the needle is worn, which a lot are from poor centerings over the years you are completely wasting your time trying to get a tickover which is not overly rich. And to set an SU up correctly you must first get the idle mixture correct, then correct the mixture elsewhere in the engines range with alterations to the needle size, NOT by raising or lowering the jet as so many people do. That approach may get you a reasonable mixture at a specific point in the range but I can guarantee it will not be optimum elsewhere, and if you do not have optimum mixture you will not have optimum performance, I don't care what anyone says this point has been proven time and time again.