[TR] zenith or su's ?

[Michael Marr]

----- Original Message ----- 
From: <jimmuller at rcn.com>
To: <Triumphs at 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