No...whilst the needle profile does of course have an effect and in days gone by, maybe even now, dedicated guys would grind their own profiles....the jet position covers the starting point for mixture across the whole airflow range.
Find following a quick guide how to set up an SU....pretty much everywhere you go will tell you the same thing, but this one was nicely written and worth saving - credit to the guy with the nice MGC...
SU's are a nice carb that will get you out of trouble even when wildly out of tune, they are fairly easy to tune and about the only things to watch are the float height and if the spindles are worn.
Looking down from the top of each carb, turn the adjusting nut on the underside of the HS6 clockwise to richen the mixture (jet moves down exposing a thinner part of the needle) or anticlockwise to weaken the mixture.
Again there are a number of ways of checking the adjustment of the mixture at tickover. The principle used here and for all of these and the standard tuning methods is the implicit assumption that if the mixture is adjusted correctly at tickover, then it will automatically mean that the mixture is correct across the whole rev range. This is true if the engine is standard and unmodified and using the standard recommended needles.
So by way of example, using ST needles in an unmodified C engine with original air filters and adjusting the mixture to be correct at idle should mean that the mixture is correct right the way to the rev limit. Obviously where you have made mods such as, in my case, Kent AH2 cam, K&Ns, unleaded head, fast flow exhaust, modified inlet manifold, 123 ignition, the original standard needles will not be the best needles for my set up, even if they can be made to idle nicely.
· The ‘By ear’ method. Sounds imprecise but in the hands of an experienced tuner, often very close to perfect.
· ‘The method in the manuals’ – where the mixture is adjusted until the engine RPM just begins to drop. If you did the 12 flats clockwise originally, then adjust by turning anticlockwise (weaken) until the engine speed just starts to drop. Then turn clockwise until engine speed starts to rise, followed by one more “flat” (on the nut) clockwise. Then, using the lift pin on the underside of the dashpot mounting flange, press up until you feel contact with the piston and then lift slightly.
· If the engine RPM increases when the pin is lifted, and remains elevated, the mixture is rich – weaken the mixture by turning the adjusting nut anticlockwise.
· If the engine RPM drops when the pin is lifted your mixture is too lean - richen the mixture by turning the adjusting nut clockwise.
· If the engine RPM initially raises as the pin is lifted and then settles back to the original RPM or very slightly above, the mixture is right.
· ‘Tailpipe Emissions’ – it is possible to use emissions testing equipment to adjust the mixture. If you have one exhaust pipe at the rear then obviously by the time the exhaust gases reach this point they will have mixed completely. So you could have a front carb very rich and a rear carb very lean and it appear to be correct. The way to avoid this is to keep them in sync. Don’t turn the front carb without turning the rear carb the same amount (if you have twin tail pipes work out which one relates to which carb). The ideal air fuel ratio is between 12.8(4.3% CO):1 and 12.1 (6.3%CO):1 but at tickover it is recommended to go leaner than this i.e. the leaner the better for the engine whilst this allows for a smooth even idle. With only a visual test for emissions, the level of CO emissions is not important for the MOT but it is for your bank balance (wasting unburnt fuel) and the environment:
· 2.5% CO – Lower end of ‘ideal’at tickover
· 3.5% CO – Upper end of ‘ideal’ at tickover
