----- Original Message -----
From: <jimmuller@rcn.com>
To: <Triumphs@autox.team.net>
Sent: Tuesday, December 01, 2009 12:00 PM
Subject: Re: [TR] zenith or su's ?
>> An Increased Spring Rate will make any specific Needle run
>> richer across the whole spectrum. No Exceptions!
>
> No argument. However:
Interesting discussion. I looked in my copy of "Tuning SU Carburettors"
(Speed Sport Publications, 1968), and the authors have this to say about
springs:
1. The rates for springs up to the 1 3/4 bore size are 2 1/2 oz (blue), 4
1/2 oz (red), 8 oz (yellow) and 12 oz (green)
2. "It should be noted that a change from a medium spring (say the red) to a
weaker one (blue), will have the effect of weakening the mixture throughout
the range. The effect of going to a stronger one will be to enrich the
mixture throughout the range."
I googled this book to see if it is still available and I found a PDF of it
at:
http://www.sterlingbritishmotoringsociety.org/files/Tuning_SU_Carbs.pdf
It's worth downloading if you are into carb swaps. There is a list in the
appendix showing the standard needle/spring combinations for most English
cars of the pre-68 vintage. The book suggests using a needle/spring
combination from a vehicle with similar carb size and similar power output
as a starting point when setting up SUs on an engine that was not originally
so equipped.
Mike
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