SU conversion

[Robin Gooders]

Hi all,

I’m thinking of fitting an SU to my car in place of the old bronze Zenith carb. I seem to remember there was a lot of wisdom on the old forum about best size to go for, needle type, spring type etc. However, now that’s gone I’ve not been able to locate any other source of info. Anyone out there got any info they could pass on please?   I’ve got a 1 1/4 “ SU and manifold which I could use, but I think I read somewhere that our cars run best on a 1” or 1 1/8” ?  Any advice greatfully received. 
Cheers, Robin

[Peter Naulls]

Hi Robin,

Hopefully someone will remind us how to access that information, as you say there is a lot of knowledge out there.

In the meantime I can say that I fitted an 1 1/8" SU to a late type original manifold and found that it ran very sweetly with minimal setting up. Needle I believe was either M7 or M9. I'm not sure what spring, probrably the lightest. (Standardish 2 bearing engine with H/C head).

Subsequently I have fitted a 4 branch steel manifold with an 1 1/4" SU; this gives more power and is free-er revving but has proved to be horribly tricky to get right. All the variables have become absolutely critical - ignition timing, plug type, petrol brand, ambient temperature etc all have to be spot on. Running during the winter months has become such a chore that I am thinking of going back to the previous set up. How much of the cause is the carb and how much the manifold I'm not sure but I'm inclined to blame the size of the carb. Needle is FI with about a thou of material removed between stage 2 and 3. The free movement of the dashpot piston is reduced to about 5 thou.

Having said all that you will read under the "leaking carb" post that another contributor has had instant success!

Peter.

[Robin Gooders]

Thank you Peter, that’s very helpful.
Much appreciated. Robin.

[Derek Sheldon]

Thank you Peter, that’s very helpful.
Much appreciated. Robin.

I have a 1” SU and would like to fit it to a standard manifold any suggestions as to setting it up would be much appreciated, jets spring etc.

[Bob Culver]

The immediate problem is sorting out the many SUs. Jeff Taylor covered this in a comprehensive article. There is little or no markings and bodies; pistons, can be and often have been interchanged. The horizontal models include 1 or 1 1/8 and are more appropriate for normal use and more vintage looking than larger semi dd. 
A very different manifold required not in keeping with earlier model Sevens. The 1 and 1 1/8 innards seem to be the same so the difference probably of little significance. 
As time progressed the axis of mount holes changed, lugs appeared for an air filter, piston material and weight changed, bore increased, and finally the light dashpot damped piston adopted, The latter on later Series E and sv Minor so relatively common and possibly the ideal. The original MOW needle in many of these a start.
If trading needle info the general installation and carb deatils (ie piston type and weight) should resonably match. 
The jet must be unworn. A lot of time can be spent sorting needles etc, esp if optimum economy sought. Anything else in ign etc not optimum can lead to extensive waste of time. 
I dunno about others but the  dashpot modesl seem to be a marked improvement, including general smoothness, cf original. Polish nicely.

[Chris KC]

I guess there could be as many replies as members!

My tuppence worth - I bought a 1 1/4" Morris 1098cc SU in a jumble sale in the 1980's for £1; took it home, cleaned it up with paraffin & a toothbrush and fitted it to my Ulster rep. I fiddled about with the idle adjustment for a while but it's been on my car ever since and runs just fine (if perhaps not 'optimally'). This particular carb is identifiable by a small triangular tab on top of the float chamber marked 'AUD13'. It has a red spring and AN needle.

I have no doubt at all this set-up could be improved on, but it provides a perfectly good runner.

My received wisdom is that a light blue spring AUC4587 and No.6 needle AUD1005 are favoured by the racing fraternity (not sure which choke size this goes with though, or how driveable it is off the track). Springs & needles can be had from Burlen or Mini Sport.com.

Chris

p.s. I just had to pop out and check, but I also have a 1 1/4" SU on my Nippy and that's an AUD431L. According to the book that's from an MG1300, and has a GY needle and blue spring; though I can't swear these weren't changed without stripping it I'm afraid. The Nippy has been lying fallow for a bit, but used to go like the proverbial clappers. Incidentally on this car I used a Reliant SV exhaust manifold, which has an integral inlet casting, and fabricated a short angle adaptor from steel tube for the carb.

[Alan]

The trouble with SU carbs is that they will run tolerably well when the setting is miles out, and because of different degrees of wear and minor differences, what works well with one may not be so good an another. I have three Austins with SUs and they all have different needles, all do what they are supposed to. My advice is unless you really know what you are doing with these carbs is to take it to someone who does. An hour spent on a rolling road will make a huge difference.

[Jeff Taylor]

The immediate problem is sorting out the many SUs. Jeff Taylor covered this in a comprehensive article. There is little or no markings and bodies; pistons, can be and often have been interchanged. The horizontal models include 1 or 1 1/8 and are more appropriate for normal use and more vintage looking than larger semi dd. 
A very different manifold required not in keeping with earlier model Sevens. The 1 and 1 1/8 innards seem to be the same so the difference probably of little significance. 
As time progressed the axis of mount holes changed, lugs appeared for an air filter, piston material and weight changed, bore increased, and finally the light dashpot damped piston adopted, The latter on later Series E and sv Minor so relatively common and possibly the ideal. The original MOW needle in many of these a start.
If trading needle info the general installation and carb deatils (ie piston type and weight) should resonably match. 
The jet must be unworn. A lot of time can be spent sorting needles etc, esp if optimum economy sought. Anything else in ign etc not optimum can lead to extensive waste of time. 
I dunno about others but the  dashpot modesl seem to be a marked improvement, including general smoothness, cf original. Polish nicely.

For a start, here's one of my posts from 2006. I haven't as yet found the post Bob is referring to, but I do remember posting it, probably in around 2013/14? I'll continue searching the old forum until I find it - it may take some time !


  Screen Shot 2017-11-05 at 11.12.24.png (Size: 162.8 KB / Downloads: 1,029)

[Bob Culver]

Having slept on it I realised I had quoted the wrong needle but Jeff has beat me to the correction. The MOW proved impossibly weak.

[Austin in the Shed]

If you have scuttle tank mounting an SU on a later manifold will leave the carb too high unless you fit a fuel pump,or you keep the tank full.
On my box I made a manifold from an updraught aluminium one and fitted a 1 1/8" which seems to work well.