how do i adjust the air/fuel mixture on my 122 with su hs6 carbs?

This is a pretty complex topic and there are a LOT of side issues involved.
For example if your throttle shafts are loose or there are other (very likely)
sources of vacuum leaks, adjustment will be for naught.
I'd STRONGLY suggest that you get the Haynes manual on SU Carbs and read it
before you attempt any adjustment. It will save you a LOT of grief!

--
George Downs, Bartlesville, Heart of the USA!

?

Vary the relative jet/metering needle position to vary the orifice size, which meters fuel as a function of engine demand and throttle position. I suggest you understand the operation of an SU before attempting adjustments...please assure also, that all else (i.e. IGN, valve clearance, throttle shaft slop etc. ) is right before you go trying to adjust the mixture to compensate for something else!...two wrongs seldom make a "good right"!

See also: http://sw-em.com/su_carbs.htm

Cheers