http://www.team.net/www/morgan/tech/tuning.html
Go to that web page, study it, download the software. It's better then
anything I can say about modifying an SU for performance.
Fwiw, I've got two HS2's from an early Spitfire on my otherwise
stock 1500. The original needles caused a good deal of
knocking. I switched to a set of richer needles, and the
knocking was greatly reduced. Performance may have
been slightly increased. Gas mileage isn't good, though
I don't know what it is. I didn't go through the steps yet
that I need to for correct needle selection, I simply used
some spares that I'd picked up at a swap meet.
As for the spring change, my understanding is that you
want the piston to top out near engine redline, at wide
open throttle. A header and especially a cam change
could cause a need for a different spring rate.
Could you diagnose a lean mixture by pulling the
choke? Maybe, but you have to be carefull doing
it because you can just as easily prove the mixture
is overly rich doing it this way.
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