SU Needles

Updated November 2013: I downloaded the following needles a long time ago, extracted from a needle selection program that no longer seems to be available. Note the dimensional errors in SM and SN from a number of sources, I give both, corrected versions at SM* and SN*.

Originally HS carbs had fixed needles and the jet had to be centred on the needle to prevent the one rubbing on the other, causing wear and upsetting the mixture. North America changed to swinging/bias needles on AUD326 carbs/18GH engines in 1969. Home market cars didn't get them until 1971 with AUD492 carbs on 18V engines. The jet bearing kits differ between the two - WZX1341 for fixed needles and WZX1442 for bias/swinging needles, the difference is that the jet bearing for fixed needles has a larger clearance in the jet nut to enable centring, that for bias/swinging needles is basically a snug fit in the nut to hold it in one position, the needle moving to fit the jet.

There is an online glossy selection/comparison program here. There is also a downloadable Windows program here, but this is less easy to use than the online. Whilst you can view .09" and .1" needles, the comparison program doesn't discriminate between them. So if you are looking for an alternative .09" (for example) in the Comparison section, and find a possibility, you then have to search for it in the .09" section, as it could equally be a .1" or a .125". Also not all needles are the same length, some are shorter, but the program indicates these needles continue at the same diameter as the last station. Use one of these and you could go massively rich at the top end. Just to add to the confusion there are a number of needles where the last two or three stations are the same diameter. There is also a lot of dross asking for details of your engine, gearbox, back axle etc., just go to the 'Technical' section.

The top box of each pair contains the needles listed in needle-name order and the second box contains the same needles in size order. The first one or two columns of numbers represent the idle position, and the last one or two are at the full throttle end.

Select your existing needle's name in the top box, read off its dimensions, then use the second box to find a needle with the variation you need. Remember that bigger numbers mean a weaker mixture and smaller numbers a richer mixture.

Bear in mind that if you want a needle that is richer at the top end as well as by selecting one which has the same idle dimensions but is thinner (richer) at the top end, you can select a needle with thicker (weaker) idle dimensions and the same top end dimensions. However in order to get the correct idle mixture with the second method you will have to move the jet down further than normal, which as well as giving you your richer top end will also have the side effect of moving the maximum throttle point further down the needle.

Please note that this resource is to help you choose an alternative needle to standard if you have a non-standard system e.g. different air filters, exhaust etc and the standard needle is giving flat-spots or other problems, and the MGOC has suggestions for various levels of modification here. For a list of standard needles, springs etc through the years have a look at Paul Teglers info.

0.090 Needles:

0.100 Needles:

Note: SM and SN reputedly have incorrect dimensions in most published sources. Claimed correct dimensions are at SM* and SN* respectively.