Filing SU needles

(Phil.Dobson) #1

what’s the consensus on the reported ability of some tuning people to be able to ‘file’ the mixture needles to improve performance on modified engines?

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(Andrew Waugh) #2

My experience is that about 5% of the modified needles have been done by someone who appears to have an idea of what they’re doing.

The proper way to do it is to lay the needle on a wood block and drawfile a flat in the range where you want more fuel.

I’ve found bent needles, crimped needles, needles with hourglasses sanded into them, needles which have been rubbed on a brick… but only a few which have a clean flat obviously done to make WOT a bit less lean.

Of the poorly done needles I’ve found, most of them have been modified down about where the car idles… SUs have screws for adjusting idle mixture.

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(tony) #3

If you wish to consider it, first acquire “How to power tune SU carbs” by Des Hammill

It has very detailed instructions on how to do it, and the reasoning

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(Ed Nantes) #4

The bloke who made SU carbs for racing here, would take your car out on a circuit he had mapped out incorporating different types of driving, He’d sit in the passenger seat with a portable gas analyser attached to the ex pipe and make notes at various rev points.
The needles are graduated with specific diameters at { I think] 50 thou intervals,
So he’d know from his readings at what point it should have a little machined off, or different needles looked up from his chart.
It worked well but then his day job had been SU carbs since cockie was an egg.
I’d be surprised if enthusiastic amateurs would achieve an improvement.

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