Fuel System

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SU Needles

The following needles have been extracted from the 'SU Needle Program' that can be downloaded from Rich Mason's site, also from this Morgan tech page which has further information on tuning (as opposed to setting up) SUs.

For an excellent Windows version of the above program by Scott Beavis go to this link. I say 'excellent' but whilst it allows you to compare two needles graphically which is useful, it is a bit long-winded to find one that is a bit richer or weaker in a specific area than your existing needle. This prompted me to write my own version which allows me to scroll rapidly through the list of potential needles watching for one which, graphically, differs from your own by the required amount in the required area. The versions I downloaded of both the above programs contain the errors in .100" SM and SN needles as indicated below, may be correct now.

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. 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.