The term CD refers to "Constant depression" and depression , in limey
talk means vacuum.
You need to know that the location of "this vacuum" is right in the
venturi throat of the carb , exactly where the needle and jet are
located.
The beauty of this type carb over non variable venturi carbs lies in
the fact that when you can maintain the same vac over the jet head,
you can then tailor a needle taper which will allow approx. a perfect
air/fuel mixture throughout the entire carb throttle position range.
SU and Zenith use approx. 220 feet per second velocity across the jet
and maintain this air speed at idle rpm right up to wide open
throttle. It IS IMPERFECTLY done but is damn close and for the most
part it is close enough.
However when tight emissions were required, they had to fine tune it
with more sophistication in the area of temp compensation and also by
the requirement of the needle itself ALWAYS being in a known position
in the jet.
This was done by spring loading the needle so it rubs the rear face
of the jet at all times.
The term altitude compensation can and does refer to more than one
problem. It used to refer only to having a atmospheric vent to the
float bowl but it now involves temperature and barometric
measurements and alterations to systems controlling actual air/fuel
ratios.
In current passenger cars we monitor lots of things like outside air
temp, baro pressure, manifold temp, manifold vac, coolent temp,
engine loading, engine rpm etc.
As a result of such wide ranging monitoring, modern cars are burning
cleanly and getting max fuel economy and..................
unexpectedly....greatly increased engine life !
When Volvo developed their first major US emission system back around
1972-75 they were shocked to find it added about 40% to engine life.
They at once made this engine their std for all world-wide markets.
The main feature of tight emission controlled engines that produced
the longer life is.....stopping over-rich and over choked fuel ratios
during cold starts, warmup , idling and deceleration modes. Rich
mixtures wash oil off rings and cyl walls. Rich also causes oil
dilution and poor lube protection.
For the CD150 carb. you need not be concerned over an alt. change in
the range you are able to sustain on the road. There would likely
be less than a 2% error from the ideal mixture going where you go.
As Jarrid has stated, there is an improvement to be found by
readjusting the mix and the rpm when your basic climate changes. As
an example: if you could sustain say 25 deg.C. change between a
winter and summer climate (on average) .3 x 25= 7.5% change in ideal
mixture ratio (approx). This is large enough to justify retuning the
carbs. Larger temp changes result in larger errors.
Rootes was in it's death throes, when US emissions became required.
The emission version of the CD carb was at best a poor attempt to
avoid fuel injection or entirely new types of carbs. Jaguar
particularly almost had to go bankrupt in the 74-76 -period when
their CD carbs caused endless problems which were almost impossible
to correct. Rootes would have had the same horrors if they had been
selling here at that time.
None of the early emission carbs ever gave suitable service and all
were unreliable (Ford, GM, Chrysler etc.) in their first formats.
Later second phase carbs did operate acceptly well and lots of older
cars still perk along today with these later designs.
------------------
Back at the time of WW-1 the Skinner-Union (SU) carb was the envy of
all european engine designers but it always has required a delicate
touch by the mechanic so as to not damage piston/dome surfaces etc.
In the 1950s I had a part time job with SU to teach carb school in
Chicago a few times per year.
The Zenith Stromberg CD type is robust and not sensitive to dirt,
grit, rough handling etc.
I still liked the old 1914 Claudel on which I was involved. My 1920
Buick K45 has the very FIRST compound carb. which is a two throat
with a primary and the secondary operating on increased air velocity,
just as we saw "reinvented" back in the early 50s in the USA.
In answer to one message received from the list, the full name of the
carb. is" Claudel-Hobson" and surprisingly....it actually found it's
way onto a Sunbeam at one time.
In 1925, Sunbeam came out with a new sports car based on their Grand
prix contender . This was another design by the chief designer for S.
T.D. Ltd , Louis Coatalen. What a great mind he had, and what a joy
to know him and absorb cutting edge info at that time.
The 1925 3.0 litre six cylinder he designed had twin OH cams, seven
main bearings, twin carbs. Cams were shaft driven by helical gears
and it was LOUD above 3000 rpm , in fact I would say it actually made
a high pitch howl sound and conversation was impossible at speed.
While the sports car was a good seller, it placed well in only one
major event....the 1925 Le Mans, where it beat out such cars as the 3-
litre Bentley..to finish 2nd . I have a scar today, from being burned
on the hot engine while leaning on the exposed exhaust pipe.
The model carried on into 1931 with good sales but not much in the
way of race major wins.
The two Claudel Hobson carbs helped produce 105 bhp at 3800rpm and
top speed for this two seat roadster was about 90-92 mph (at a time
when 70mph was considered a VERY fast car).
Naturally, I was involved with carb services and training during this
period.
In 1928 Sunbeam adopted a supercharger on the 3 litre , for race
purposes. This was a bad idea and repeated failures resulted in races.
One built up for Malcolm Campbell and one for a Mr. Davies both
disintegrated in their debut events in 1929 and 1930. The blower was
at fault both times but also in the 1929 car of Campbell, the clutch
failed to handle the 138 bph we generated.
This pretty little car with equally pretty DOHC engine went out of
production in1931 and the co. had NO sports car at all until the
reorganization in 1932 gave us the Dawn model of 1627cc displ. which
was push rod operated.
By 1935 Sunbeam finances were so bad that the co was acquired by
Rootes organization who were the principal creditors. Rootes also
picked up "British Talbot". The new firm then bore the name Sunbeam-
Talbot.
BTW.......perhaps some of you on the list would like to know what the
word Talbot means??????
It is a very large hunting dog (French) used to bring down stag and
other game. You see it depicted on the old logos of the co and it
looks like a lion with a crown but it is actually the huge hunting
dog.!
Dick T.
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