[TR] zenith or su's ?

[Wayne Lee]

Hi Mike,
    Yes, that's what I meant when I wrote:" An Increased Spring Rate will 
make any specific Needle run
 richer across the whole spectrum. No Exceptions! "

That means even if the Needle is the wrong profile and runs leaner than it 
should at 75 mph but isn't bad under
2500 rpm's, the heavier Spring will richen it up evenly across the board. 
Maybe making it the proper Air/Fuel ratio
at 75 mph., but making it run in an over enriched mixture when running again 
below 2500 rpm's.

    The idea is to get it running at as good an Air/Fuel mixture at all rev 
ranges, then use as light a Spring as necessary.
When everything else is ideal except maybe getting a lean Pop at higher 
rpm's., the next highest rate Spring could be
just the ticket. When You have large contrasts in Mixtures over the Rev 
Range, You fix the Needle profile first as a Spring change
alone isn't going to do it. Modifications such as Camshafts with longer 
duration, Headers, C/R, Free Flow Exhaust and Air Filters
can alter the needs over a Stock Needle application. Conversions only 
complicate things further.

I have the Book You mention, but I always like them in PDF Form as well so I 
can check them out here at work while the CNC's are
running and nobody knows what I'm really doing. Thanks for the link, I'll 
download that.
Wayne


--------------------------------------------------
From: "Michael Marr" <mmarr at notwires.com>
Sent: Tuesday, December 01, 2009 3:19 PM
To: <jimmuller at rcn.com>; <Triumphs at autox.team.net>
Subject: Re: [TR] zenith or su's ?

> ----- 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