In a message dated 97-10-27 13:18:48 EST, Dave_Rupert@codl.com writes:
> Isn't it the other way around? The oil is supposed to work as a
> damper, so getting rid of it would make the piston move more quickly.
Dave:
Yes, that is true, the oil does act as a damper. This prevents the piston
from rising untill the engine rpm has had a chance to rise also to increase
the air flow. If the piston rises too quickly, creating a larger venturi,
before the air flow catches up, there is a momentary drop in the vacuum
signal to the fuel supply, which cause the carb to go lean - the opposite of
what you want when you step on the gas. Under sudden full throttle condition,
the engine needs a richer mixture than normal.
By delaying the piston rising, the carb intake is suddenly exposed to the
full vacuum of the engine, and, because the piston has not yet moved up, a
higher than normal vacuum exists at the needle and the fuel jet, sucking more
fuel in than would be the case under steady state conditions. This gives the
richer mixture the engine needs at this time.
The CD in the nomenclature for these carbs stands for "constant depression,"
depression being the British term for vacuum. During operation, the piston
floats up and down as required to maintain a constant vacuum signal to the
needle and jet assembly. A large opening with high air flow gives the same
venturi effect as a small opening with low air flow. Because the needle is
tapered, as it rises it effectively makes the jet opening larger. Thus, with
the same vacuum, or depression, more fuel is drawn into the intake.
There is a trade-off involved here. Suddenly raising the piston increases the
jet size, but the reduction of vacuum is more than enough to defeat the gains
from enlarging the jet.
In an American carburetor, when you stomp on the gas pedal, there is a
physical pump that squirts an extra shot of gas into the engine to give a
momentarily richer mixture.
This is hard enough to understand, but danged near impossible to describe
very well. At least the above is my understanding of how a CD carburetor
works. However, since there are no wires involved, I make no claim to any
real knowledge!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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