I've been playing with needles and mixture settings for the last 6 months on my special. I started by reading most of the info on this forum and any info on the net, I selected the weakest needle I could find with the light blue spring in a 1 1/4 SU. We fitted a lambda sensor and gauge to monitor fuel mixture. The setup was good but a bit rich on tickover about 10:1 better on acceleration about 14:1 but on crushing on light throttle the mixture would weaken out erratically and start to misfire. I tried different needles, filling needles no spring, cut down spring.

I was getting a bit fed up with this and decided to try something different and fitted a stiffer spring. Mixture on tick over is still a bit rich 10:1 but ok on acceleration and good on crushing 16:1.

Not sure how fitting a stiffer spring works but it did.

Hi Leon,
the standard design spring for S/D and semi S/D SUs of about the right size for the engine is RED 4.5 oz. Usually works out reasonably with something not too different from an AN needle with a 1 & 1/4" HS2.

Ian Mc.

Location: Shropshire

Leon - that is practically my experience, without the posh sensor!
I am going to send this thread to Burlen for their comment.
On reading everything, I'm also going to try the advice from David Wallom on another old SU thread, which just seems to have some possible relevance in that 'momentary' point that is resulting in the difference of interpretation.

"I have a fairly standard 3-bearing engine with a high compression head. I was finding that pickup was very poor but would run fine once the engine was revving. It has an AN needle and a blue spring.
The problem and hence solution was that when you first blipped the throttle from idle the piston would rise very quickly causing rough running. To compensate for this I had set it at '14 flats' i.e. 2 & 1/3 turns from level. This rapid rise is caused as the damper 'float' being free to within a small measured amount on the end of the stick. once it rises above this amount then it is working against the damper oil. I am not certain but for an engine in fairly standard tune an 1 1/4" carb is quite large, therefore the rapid rise under no damping is causing the mixture to weaken too much. It was also causing the plugs to oil up when idling and the engine to stop running and be difficult to restart (as anyone who saw me at the VSCC New Year Driving tests will assert).
I had a spare carb and removed from the damper stick a small brass washer and introduced this above the damper float alongside one that was already there. This reduced the free play of the float by about half. I then turned the mixture back to 12 plats (2 turns). The engine is now much more responsive to the throttle from idle, doesn't oil up the plugs and when testing around a car park was much more responsive etc.
So to recap, I have found success (and I am not saying this is the only way) with HS2 carb (30degree up angle), AN needle, light blue dashpot spring and 2 x small brass washers on the damper piston rod to reduce free movement."

I am wondering whether your problem may be due to the high compression cylinder head, especially if you are experiencing plug oiling.
It is very tempting to fit a high compression cylinder head to a basically standard engine as 'it will produce more power'. I once fitted a late Ruby head to my RN saloon and the engine would hardly run at all, due to plug fouling. The earlier head with 18mm plugs is much more forgiving. It might be worth fitting a lower compression cylinder head to see if the problem persists.

Other variable's are where in the rev range we are talking about mixture strength, and if you run any type of air straighteners. At low piston lifts the air flow is very much across the jet but at high piston lifts the air velocity is highest in the centre of the choke not across the jet so effective draw differs. Air straighteners can help with this by reducing break away around the comparatively sharp corners of the standard carb intake flange and evening our velocity in the throat. I spent some time on a dyno playing with piston weights (springs), needles and radius at throat, in my car increasing piston weight (spring tension) by 2oz caused significant weakness at high RPM. This effect may not be so relevant in a less highly tuned road car running with comparatively low piston lifts.

Location: NZ

Is there a difference in running an 1 1/8" SU with an air filter? The Chummy SU is fitted with a small trumpet and the Sports SU has a small pancake air filter?
The Sports seems to run more efficiently, but as I mentioned I am no expert and I don't know what needles are fitted (engine rebuild expert David Lowe set them both up originally).

Tony.

I neglected to mention both SU's are post war side draught type, I assume from a Morris Minor although the bodies look slightly different, with standard pistons, dampers and no springs.

Tony.

Hi Leon and others

Somewhat contrary to the Seven spirit, many have introduced new fangled electronic regulator and ignition systems to solve perceived problems, but there has been little about oxygen sensors, which, with so many alternative carbs, would seem to have at least equal justification. Even with everyday use, sorting of optimum carburation can be a mission.

Details of any successful installation, type and location of sensor, wiring of etc may be of interest to many. Presumably can be temporarily monitored with a digital multimeter. Although observation can be a problem in a Seven. (I fitted a volt meter below the dash to help manage the battery, but with unextended scale found I could seldom take my eyes off the road to properly read it!)
Economy is not now the key issue for Seven owners. My unfortunate experience with other cars is that seat recession is very related to weak mixtures. Seven valve seats, with no cooling duct on one side, must run very hot.

It is curious that supposed authoritative sources differ on the effect of springs.

At high air flow, air filters do somewhat richen mixture with externally vented carbs, as the SU. Many old manuals gave different jets for oil bath or mesh air cleaners. One great merit of the SU is the ability to lower the jet and richen the mixture when underway (except perhaps with EK needle)

Bob Culver

Location: Auckland, NZ