RAY WYGONIK wrote:
>
> I picked one up from TRF as a freebie about 18 months ago and it to came
> without instructions. With my 72 mgb getting close to being a road car
>instead
> of a project car, I am in need of the instructions. I haven't tuned Dual SU's
> since Armstrong walked on the moon and if my memory is right, I wasn't very
> successful. Any help would be appreciated.
>
> I have been contemplating a Weber Carb because of the troubles I remember with
> the SU's, but from the postings, the Weber aren't necessarily an improvement.
> I plan now to start with the SU's.
>
> Also, what is the difference between a flat and a turn?
Ray: S.U. carbs are a very reliable item provided they are looked after and
serviced
when needed. It is not much good trying to get an S.U to work properly by
tuning,
either with or without a Colourtune, unles it is in decent condition. I won't
now
embark on a lengthy discourse as to how to rebuild an S.U. as it has all been
said and
done before, both here and in print. Most books on S.U.s give clear details.
The turns or flats referred to are the amount the spring loaded brass jet
adjusting nut
is moved up and down the jet locking screw, in order to either richen or weaken
the
mixture. Remember also, the position of the needle in the piston and also the
level of
petrol in the float chamber both have a bearing on the mixture supplied to the
engine,
as well as the level of the jet set by this nut. On a rebuilt carb. it is
customary to
start adjusting the mixture from a nut position one full turn (or six
'flats')below it's
tightest place at the top of it's travel on the jet locking screw. When
tuning, it is
as well to adjust a little at a time, hence, turn the nut one 'flat', six
'flats' being
equal to one complete revolution of the nut. This makes a very small
adjustment in the
mixture which, although a more prolonged process, will result in a greater
degree of
accuracy.
I strongly suggest you refer to the excellent article by Mr. Chip Old in the
NEMGTR's
digest of technical articles called The T-Series Handbook, page 117, Back to
Basics.
Especially see figure 17 on page 126. This article says it all.
Geoff Love.
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