Been wracking my brains as two learned counsel on the forum - who have both given valued time trying to solve problems of mine beyond the call of duty - have opposing views on this.

I thus specifically bought Hammills's book, and contacted SU/Burlen to double check. Both Hammill and Burlen disagree with the 1970s book Bob quotes above i.e., they say that a STRONGER spring actually weakens the mixture and acts as an economy device.

I am left with an inability to correctly diagnose the issues changing from red (good) to blue spring (no good) until I get to the bottom of this! The other ambiguous factors are that it is universally said that 'a stronger string will harm the acceleration' which I presume doesnt actually need to the be same issue as the mixture. Ditto, 'use of a stronger spring may prevent flat spots by the piston opening too fast and weakening the mixture' This latter point starts to suggest something which links the two opposing camps, but my brain starts to fade in trying to work it out! Annoyingly, Hammill, whilst giving some great further advice on things like limiting piston travel and boring out the hole in the damper piston top, doesnt actually answer the naive question of how exactly these issues interact.

Can anyone come up with a uniting theory? There are elements of rightness in everyones' views, but we cannot get away from the fact that two key texts on SUs have opposing views!

Think about the damper, it's point is to slow the piston when you crack the throttle open, by holding the piston down the mixture richens briefly in the style of a pump jet. So yes, a stiffer spring will richen the mix.

Location: Oxfordshire

Jon,
It is to do with the differences of inertia between fuel and air, fuel being the slower to react to pressure differential when the throttle is opened. The damping effect only monetarily richen's mixture restricting air flow as fuel flow catches up, after that a lower riding piston weakens mixture.

Location: NZ

I hesitate to enter into a lofty discussion by experts in the SU debate but if the SU needle was a continuous taper, I would assume that lifting the piston/pin assembly would allow more fuel to flow. It would then follow that a heavier piston/stronger spring would reduce the lift of the piston/needle for any given vacuum and reduce the fuel flow.
I understand that the very mysterious variations in the needle taper will then produce many variations in fuel flow with piston lift.

I will go and lie down now !

Tony.

There is also another factor operating.
Petrol doesn't expand and compress as the vacuum changes. Air does.
This makes the metering complicated during throttle opening as the vacuum in the carburetor changes rapidly.
It's handled by the compensating jet in a Zenith, but by weight, springs and dampers in an SU.
The object is to try to keep the air/fuel ratio constant at about 11:1 by mass. Not that easy!

Location: Melrose, Roxburghshire

Doesn't it depend on the variation of taper of the needle as to whether is richens or weakens the mixture? On a needle with little taper a strong spring might richen it but with a steep taper it could weaken it.

"Basic Principles of the SU Carburetter

The majority of car engines rely on carburetters to provide them with the finely atomized fuel/air mixture necessary for satisfactory performance. The carburetter must provide good atomization and the correct mixture strength under all operating conditions of the engine. The method used to do this in all carburetters is to speed up the velocity of the air by means of a venturi or choke and to use the consequent reduction of pressure in the venturi to draw fuel from the float chamber through a suitable jet orifice into the air stream.

The ideal carburetter is an instrument which when correctly tuned will supply its engine with the optimum mixture for maximum power throughout the full throttle range and for minimum consumption under all part-throttle conditions.

The Fixed Choke

When air is passed through a choke of fixed size its velocity and the depression over the fuel jet will vary with the demands of the engine. This varying depression makes it necessary to employ compensating devices to produce the correct fuel flow and also imposes a compromise on the choice of choke size in that too small an orifice will produce a restriction, at the top end of the output range whilst a large orifice will cause poor metering and indifferent carburetion at the lower end of the range.

The Variable Choke

The principle of the variable choke carburetter is to employ a means whereby the effective choke orifice will expand as the demand increases, and contract when the demand diminishes. Such a variation in choke area will achieve a constant air velocity and depression over the jet.



Basic Construction

A variable choke orifice is obtained in the SU carburetter by the vertical movement of a close-fitting piston (1) positioned above the fuel jet in the centre of the body casting. A suction disc (2) is integral with the piston and works in a concentric chamber bolted to the top of the body casting.

Drillings in the under face or side of the piston, shown for simplicity as an external duct (3) communicate any depression existing in the space between the piston (1) and the throttle disc (4) to the chamber above the suction disc (2). The underside of the suction disc is vented to atmosphere or to the air cleaner by transfer holes located in the inlet flange, shown simply by the drilling (5).

As the choke orifice is varied over wide limits by the movement of the piston throughout the speed range, the fuel jet orifice must also be varied. This is achieved by means of a tapered needle (6) attached to the piston and projecting into the jet. Correct discharge areas are obtained by the accurate dimensioning of this needle.

Opening the throttle disc (4) allows the manifold depression to be communicated to the body of the carburetter and also to the chamber above the suction disc. The piston will rise, allowing a mixture of air and fuel to pass underneath it to relieve the depression. The piston will continue to rise until the depression has reached a value which is just sufficient to balance the weight of the piston, together with the load exerted by the piston spring.

It will be appreciated that approximately the same depression can be obtained whatever the demand and that the piston height will be governed by the mass of mixture flowing beneath it. This depression is arranged to be of sufficient value to ensure that good atomization is obtained, but small enough to ensure adequate engine filling at high speeds."

Above is an extract from a SU Carburettor Co. paper. Read it a couple of times, especially those sentences in bold and I will attempt to explain why a weaker spring results in a weaker mixture. "I will say this just the once........"

The basic principle of the SU is that of maintaining a constant level of pressure - below atmospheric - at the face of the jet across the complete range of engine demands for fuel.This more or less constant level of suction at the face of the jet ensures that the petrol released into the air stream over the bridge of the carb. is varied entirely by the dimension of the needle at that point in it's travel.

So far so good; as the above excerpt states in the penultimate paragraph "The piston will continue to rise until the depression has reached a value which is just sufficient to balance the weight of the piston, together with the load exerted by the piston spring". Now, from this it should be understood that a lighter piston and/or a weaker spring will result in the piston rising higher than before for a given demand by the engine. By so doing the cross sectional area of the choke below the piston is increased and thereby the depression (or "suction") at the face of the jet reduces. Reduced suction here results in a weaker mixture for a given needle dimension.

Ian Mc.

Location: Shropshire

Which just goes to show what a wonderful device the SU carburettor is!
Like the old Chinese proverb about the man with a hole in his pocket, with an SU he will have something to play with all day.

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