Setting SU Mixture

[Ian Williams]

If your carb has ever be used without the needle centred the jet will almost certainly be worn, if this is the case, as it probably is, you will never get the idle mixture correct. The problem with an SU is that the wrong needle can be made to work by adjusting the height of the jet so mixture is correct at cruising speed, however almost everywhere else will be a compromise. Most SU needles are identical at the first two measuring points, to set the carb up you should get the idle mixture adjusted first, if the mixture is incorrect at any other measuring point a different needle should be selected to correct this, NOT by raising or lowering the jet, a flat or two is probably acceptable but will still compromise idle. Where people fall down is recognising where in the rev range you are week or rich and which measuring post this equates to, however getting it exactly right pays benefits in overall performance, particularly noticeable with acceleration.