(not an expert, just a hacker)
Ray:
A widly varying idle is usually an air leak problem. Make sure the
intake manifold is torqued down to spec. On the MGB with a separate
intake and exhaust manifold, we had to make stepped washers to compensate
for the difference in thickness. Air Leaks tend to raise the minimum idle
speed and force you to over-rich the mixture. Some of the air is unmetered
and so the marvelous constant depression SU technology is bypassed...
causing varying mixtures at different throttle settings. Anyway, this
is extremely common when putting things back together... Spray Brakekleen
all over the carbs and intake manifold gasket and see if the idle changes.
Have the throttle shafts been rebushed lately? Spray there especially.
If the idle is just high, your throttle linkage is out of adjustment,
and the throttle plates aren't closing all the way. On both the Spitfire
(dual HS2s) and the MGB (dual HIF4s now) there are adjustment nuts for
each linkage. It's easy to check, just loosen the adjustment nuts and
try to close the throttles further without extreme force.
If you are using extreme force, maybe the throttle plates are not the
right size/shape/orientation? Take a carb off and see if the plate closes
"pretty much".
I make an increase of 23% in cross-section between an H1 and an HS2....
aside from any improvements in ancillary stuff (float bowls, etc) this
could be nice at the top end (I would expect most improvement to be seen
at high RPMs). 23% cross-section -> up to 23% more airflow -> up to
23% more HP -> up to 7% increase in top speed (whee).
As a contrast, the typical MGB upgrade would be HS4s (1.5") to HS6s
(1.75"). This is a 36% increase, and is supposed to have few negative
effects. I wouldn't worry too much about slowing the airflow, especially
with your tweaked engine. Try to avoid HS6s on the Sprite though <g>.
You may need a more open exhaust to see any benefit to this... depends
on where the bottleneck is in your system. Flow in ~= Flow out
(neglecting oil, blow-by, and black holes).
You'll get some more torque (and burn more gas under WOT) at low end.
Exploiting the top end will require getting nearer to the redline... (7%
higher RPM) and here is where your rockers and valve springs have to be
up to the task.
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB E Stock | '69 Spitfire E Stock | '88 RX-7 C Stock
|