Have you re-jetted your HS4's? I've seen some impressively hot B motors still
running HS4 carbs, with simple and cheap needle replacements. A #7 is usually
quite enough to keep a 3/4 race cammed, high compression, ported B motor more
than happy. HS6's cost more, need different manifolds, and will destroy your
low-end response and fuel consumption. When you look at the B motor as an air
pump, the amount of airflow available from the 1.5s should be more than
enough.
Peter Burgess' dyno results in his Power Tune Your MGB book doesn't recommend
1.75's until you're working with a fully improved "big bore" 1.95 liter
motor, and there they don't show more than a 3% power gain with a 280' cam or
greater, and then only above 6000rpm. If you really feel you must tweak
beyond needles, he shows throttle shaft and flow mods that will increase
airflow by over 12%... most of the way to the 1 3/4 flow.
As far as the Hitachi's go, they were built under license from SU, and are
functionally clones of HIF6's... except for the fact that you can't get any
parts for them at all......
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Date: Tue, 2 Nov 1999 16:49:48 -0800
From: Larry Colen <lrcar@red4est.com>
Subject: SU substitutions
My HS4s are playing the leaky throttle shaft game. I think
that if/when I replace them I'd like to go up to HS6's. I'm
willing to sacrifice a little bottom end for the better top
end breathing.
First of all, does anybody have an 1 3/4" SU manifold for
an MGB that they'd be willing to part with for a reasonable
price?
Second of all, are there any HS6 clones that are superior
in performance or durability to the original SUs? Specifically,
are the Hitachi's on Z cars any better? What cars in junk yards
would be good donor cars for carbs, or where should I go for
a new set? Who does a good job of rebushing SUs?
For that matter, does anyone have a set that they'd be willing to part
with cheap?
Larry
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