>From experience with an HD8 SU carburettor, I can confidently tell you that
up to around 8,000 feet, it is fine to lift the mixture nut up two or three
flats. (IMHO the correctness of the mixture is better determined by
examining the colour and state of the spark plugs.)
If you lift the nut much more than that your idle and initial acceleration
take place using the wrong part of the needle, so if you want to go above
10,000 feet or so and obtain optimum performance you must fit a thicker
profile needle. As the SU H(S)6's work in exactly the same way I'm sure the
same applies.
Remember that if sea level pressure is say a nominal 1,000 mb, the ambient
pressure at 10,000 feet is around 500mb. So if you don't change the amount
of fuel going into the engine with altitude the mixture will progressively
richen until there is a serious degradation of performance.
David Brister.
1967 TR4A.
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