Bert Otten wrote:
>
> I don't have any clue to the symptoms George Merryweather described, -
> as he seems to have checked about everything - but while we're on the
> subject, does anybody have a SU needle chart to explain the difference
> in needles to be used?
>
> Here's the problem: I have the twin SU HS4's with FX (instead of No:5
> needles) and K&N air filters. Otherwise standard engine. Someone
> suggested to use the richer needles, No:6, for this setup. I can follow his
> advice, since I have a slight lag in acceleration between 1500-2500 RPM.
> Richer needles could cure the problem.
>
> But what is exactly the difference between FX, No:5 and No:6 needles?
> Anyone?
>
> Thanks,
>
> Bert Otten
> '66 B
Bert: What a good question! However, I think you must consider if you
install "richer needles", will those needles be satisfactory outside the
rpm range with which you are concerned? No doubt you have the Haynes
manual "SU carburettors". If you look on page 111 you will see the
listing for FX needles. Presumably, the smaller the diameter of the
needle, the richer will be the effect ACROSS THE WHOLE RPM RANGE. As is
my understanding, SU needles are far more complicated than just that.
They change diameter in DIFFERING AMOUNTS at the SAME points along their
length, when comparing two needles of disimilar types, the idea being
that by varying not only the number of one type, but also the type
itself, a person setting up an SU can adjust the mixture over the whole
of the operating range of the motor. For example, a given engine may
require a certain needle to give satisfactory performance in, say, the
idle to 2000 rpm range, but require a greater enrichment of the mixture
ratio over say,2000 to 3000 rpm. than one particular needle would
permit. As I understand it, by changing not only the numerical value of
needles within a given type's range( resulting in an equal enrichment
across the whole of the operating range), but also the type of needle,
it is possible to alter the mixture ratios at different rpm ranges.
Unfortunately, I haven't been able to find any definitive information
which will tell me what is the diameter of one needle AT A CERTAIN
GIVEN POINT ALONG IT'S LENGTH, when compared to another needle. If that
infomation were available, I for one would like to see it and if anyone
on this list can point me in the right direction, I shall be very
grateful. I run a 52 TD which is fitted with a Shorrock Supercharger.
I have difficulty in finding out how to satisfactorily adjust the
mixture across the whole of the rpm range which I allow my engine to
give me, other than experimenting with many different needles, which is
not only time consuming, but also costly, bearing in mind that when I
have got it right, I shall only require one needle.
Geoff Love, The English Connection.
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