[TR] zenith or su's ?

[Wayne Lee]

I'm always aware of the "Point of View" element. That's what opens the door 
to these ill perceptions.
An Increased Spring Rate will make any specific Needle run richer across the 
whole spectrum. No Exceptions!

  Another common reason for this particular Conversion resulting in too rich 
conditions is making sure the fuel Bowls are
the correct ones designed for the Spit and MGC. (AUD 2140) Just because you 
get Fuel Bowls that put them vertical
doesn't matter, if You use the taller (AUC 1310) it will result in overly 
rich conditions because of Fuel levels in the Bowls
that can't be corrected by Float level.

  If You are running the Stock Springs from a Donor Carb they will have most 
likely at least the "Red Spring" 4.5 oz
The Blue Spring(the lightest from the Midgets and spits at 2.5 oz wouldn't 
kikely be on HS6's. You don't get into the Yellow
or Green Spring Rates 11 and 15 oz respectively until You get into Healey 
territory which requires a richer mixture.
 You don't want to compensate for bad Needles with Springs. The Needle 
Profile or what Jim called "calibrated" is the
most important part. Sort the Car out on the road using the Volvo Needle 
then see where you are rich. At idle or popping on the Highway.
Then take it from there. The Springs should be your last concern.
Wayne

--------------------------------------------------
From: <jimmuller at rcn.com>
Sent: Tuesday, December 01, 2009 10:46 AM
To: <Triumphs at autox.team.net>
Subject: Re: [TR] zenith or su's ?

> Wayne Lee wrote:
>> But, Actually it's the other way around the softer Springs
>> will let the pistons rise too soon as You say, but this
>> makes the mixture too lean.
>
> Ah, there is a wildcard here which hasn't been mentioned.  It's a question 
> of point of view, really.  Let me re-phrase my statement:
>
> It depends on how well the needles are calibrated for the airflow rate. 
> With fixed venturi and jet size the mixture normally gets richer as 
> airflow goes up.  So if the needles are chosen right for the flow rates 
> for which the piston is still operational the mixture will begin to get 
> richer as the airflow rises above that rate which tops out the piston.
>
> Whether it runs rich or lean below that point depends on the needles and 
> idle setting.  You normally set the mixture at idle, but that may or may 
> not get it right for part-throttle.  You might notice lean running at 
> part-throttle, and adjust them richer to compensate.  That may even 
> require too rich a mixture for idle.  But the real point is this - it will 
> undergo a mode change between flow rates above and below the rate which 
> tops out the piston.
>
> On my own GT6 a PO had converted to MGB HS4's without changing the 
> springs.  Just as you say, they tended to run lean when the mixture was 
> set properly at idle.  Tweaking idle mixture and swapping needles could 
> make it better, but the truly confusing behavior was that at full-throttle 
> it screamed.  I kept asking myself how could it run so well at 
> full-throttle while so lean below that?  The not so obvious explanation 
> was the mode change to richer once the pistons topped out.  Once I 
> replaced the springs the rest of the behavior fell into place.
>
> Jim Muller