Common carburetor of 2CV and Visa is the Solex 26/35 CSIC type. '26' and '35' are the dimensions of the trottle blade diameter.
Beside small details changes this carburetors are of the double downdraft differential opening types. Citroen manuals and repair manuals from other publisher indicate how this carburetor work.
The following show the types with increasing power delivery:
|Type||Ø||Free area in cm2|
|2CV||26/35 - 18/26||7,85|
|Dyane6||26/35 - 21/24||7,98|
|Visa||26/35 - 21/26||8,76|
Not to forget that the manifold pipes have an inner diameter of approx. 32-33 mm,
a free area of 8,45cm2.
That show you that the VISA carburetor is the greatest possible one!
No great advantage is get by using a carburetor with a greater area: The "resolution" will be worser, meaning that the carb is not able to deliver an adequate fuel/air ratio mixture over the entire RPM range. Rejetting and tuning of too big carburetors will be very difficult due to a too slow air velocity in the venturi area.
Free breathing of engines is only possible with bigger diameter of manifold
pipes or with single carburetor per cylinder. Last method is the common way in
engine uprating: How to make real power in an engine ? Simple :
With a carburetor for each cylinder!
We build this twin carburetor kits in the 1970's in a few numbers and a second time in the 1980's when the 24 Hr 2CV races in Spa/Francorchamps began, later also for UL- aircraft engines based on the VISA-Twin.
The twin-kit contain
2 carbs at your choice (28 or 30 mm in diameter at venturi), 2 short aluminium
casted and machined manifolds (inclined at 30° upward), command cables
for throttle and starter, air filters.
The short manifolds where developped on a flow bench to make sure that the flow is not disturbed to provide a maximum of possible cylinder filling. They are well suited for racing application.
The twin-carb kit can be delivered with following types of carburetors:
Slide carburetors are
lightweight and cheap. But have the disadvantage that at low RPM, WOT and
under load the carburetor dont "see" enough air speed to mix the right
air/fuel ratio in the venturi area, with the following effect: "Blop-Blop..."
Is what you hear when the engine quit! No fuel, no power!
This engine diying can be counteracted with the right feeling in your foot during acceleration at this moment. At race track the engine dont revolve so slow that the above "Blop-Blop" sound appear!
relatively heavy and expensive. Their great advantage is to deliver allways
an relative adequate air/fuel mix at any engine speed : A depression triggered
device adjust the right air speed in the venturi area to perform
the adequate mixing of fuel and air.
This all allow to a smooth driving of your car.
Above mentioned carbs
are normally build for motorcycles. When feeded with fuel under pressure
from a fuel pump they have the tendency to overflood. In cornering the
float bowl tend to be not filled by fuel why the floats are blocked in
their free movement due to centrifugal forces.
We use only special developped carburetors for aircraft application, that are well suited for car application.
Not necessary to mention that the exhaust system has also to "grow up" in volume flow and backpressure reduction, not a theme in a racing car!