In response to Gus' inquiry re carb styles.
I am not an _authority_ on SU's but have gleaned the following in my
experience.
The fundamental performance of all SU's is essentially identical, with
some variation depending on throat size (a fixed property, obviously),
spring strength and needle taper (both variable, obviously). This
flexibility permits their adaptation to a broad variety of
applications.
H4's (installed on MGA's) have the primary disadvantage of feeding
the jet through a side hole. Since the jet must slide to gain choke,
this requires both top and bottom seals to keep the go-juice from
leaking out. These seals, usually cork, are prone to wear and drying
out, resulting in a goodly amount of petrol dripping onto your heat
shield. Not desirable.
HS4's (on B's through 71 or so) are essentially the same carburetter,
but have a hose from the float bowl to the bottom of the jet,
eliminating the need for the leaky seals. The jet is still moved down
to gain choke. The drawback is that neither do these hoses last
forever, but they can go significantly longer than the H4 seals.
The "IF" in HIF4 (fitted to 72-74 B's) means Integral Float chamber.
The bottom of the jet is simply dipped into the actual float chamber.
Movement of the jet is no longer used for choke. The choke enrichment
comes from an auxiliary valve. This permits the addition of a
temperature compensation mechanism to move the jet up and down to keep
the mixture less dependent on outside temperature. This latter
feature is one of two concessions to emission control. I am not of
the opinion that it penalizes performance, and some have suggested
that it actually permits more precise tuning. The other concession is
the poppet valve on the butterfly, which has already been discussed.
Which to use? I don't see much difference between late HS4's and
HIF4's, especially if the latter are equipped with solid butterflies,
which are easily available from the usual sources. I say late HS4's
because they are equipped with a spring-loaded (as opposed to
fixed) needle, eliminating the need for alignment, which is tedious
and tricky and probably wrong on about half the SU's out there. In
theory the HIF's should last longer. You don't have to replace the
jets every so often due to hose wear or failure, and they have those
nice seals on the throttle shafts to mitigate the effects of the
ubiquitous wear. Your bigger challenge is figuring out what piston
spring and needle to use in your application.
As for brass or aluminum for an adaptor, aluminum is easier to work
and should have no problem in that application. After all, the
manifold is aluminum.
Good luck! A. B. Bonds
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