Hi Shawn and list,
> Adding a third
> identical carb to a car desinged for two should theoretically have an
> over carb effect.
I don't think this is true for a Constant Depression carburetor. The air
values rise and fall on the CD carbs so as to maintain a Constant
Depression -- a constant vacuum. If there is more demand for air/fuel
the air valve rises to supply the additional volume. In a very real
sense this means the CD carb automatically adjusts it's throat size to
match the demand of the engine.
If you add more CD carbs to an engine, the air values in each one simply
ride lower (smaller throat) because now there are more carbs supplying
air/fuel to the engine, and each one has to supply less of the total. If
you reduce the number of CD carbs, the air valves in each one ride
higher (bigger throat) because each must supply more air/fuel.
It says to me that the only reason to have triple ZS is if the dual ZS
do not supply enough air/fuel at WOT.
In all cases it seems to me the air/fuel ratio is the same. The air/fuel
ratio is determined by the depression/vacuum which is a constant
regardless of how many carbs there are because the air values are
rising and falling to maintain a constant depression/vacuum. Of course
the needle/jet opening determines the air/fuel mix per the
depression/vacuum, but the depression/vacuum is a constant regardless of
how many CD carbs there are.
(acceleration is not address in the above)
If the above is not true for some reason, I hope someone will chime in
and educate me/us.
Also....
I have something of a theoretical question/comment which was prompted by
your post. I hope you don't mind me interjecting. It may or may not have
something to do with your/our understanding of your problem.
> The needle profile is designed around the
> volume of air that must move through the carb body as demanded by the
> engine. The B1AF is designed to feed three cylinders with that given
> amount of air. Under immediate load the demand for fuel from the three
> cylinders is greater than it would be for two but it is still tying to
> feed fuel for three cylinders.
I don't know if the following is true or not. I have often wondered
about it. I would be very interested in comments on it.
I believe there is only one cylinder on the intake stoke at any given
time whether the ZS carb is supplying two cylinders or three cylinders.
It leads me to believe that if the carb can supply sufficient air/fuel
to the intake stoke of one cylinder, the same cab should be able to
supply adequate air/fuel to up to 4 cylinders or until more than one
cylinder is on the intake stroke at the same time.
It would lead you believe that a 4 cylinder engine only needs one carb
and that if it is large enough to satisfy one cylinder it can satisfy
four, because no two cylinders are on the intake stroke at the same time
in a 4 cylinder. Given that there are lots of 4 cylinder engines with
two carbs, there must be something missing in my thinking.
Would be nice to know what it is :-) Perhaps valve overlap or something
like that? :-) Maybe I could sleep better if somone would tell me :-)
Don Malling
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