[TR] zenith or su's ?

[jimmuller at rcn.com]

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