Filing SU needles

I will try and upload some pages from his book on the process via pics.

Its an eye-opener

I agree its not stepped, but he uses thin strips of abrasive and the marked card to make the alterations

My 120 has 2 HD8’s. It had UO’s, but when I took it to the rolling road they filed the needles and got more power and torque. They recommended to me UN’s as a test, which they did not have in stock to try. Did not seem any difference, so assume they are also about right on my car. I had a set of spare UM’s which are the standard needles on the HD8 on an E type. Tried them - totally useless as would not rev above 3000. Also had spare UE’s again above 4000 flat.
The other thing, is it depends where jet is set, how high or low within reason.

Nobody has mentioned the graphical needle chart by the strangely named - Mintylamb.co.uk/suneedle it is very good for visualising the needle profiles from the SU needle chart booklet.

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Indeed, but that adjustment is supposed to be used exclusively for idle mixture. But as you say, many folks wind up lowering the jet to a proper mixture at speed, even though that makes the idle mixture too rich.

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Also seen people doing the opposite and put the needle slightly higher up in the piston, rather than setting the needle groove or shoulder to the bottom of the piston.