Re: Suzuki 450 GSL 1982 running lean question

paul c wrote:

Just wondering if there is a typo':

Should the above say "constant VELOCITY diaphragm ..."?

This question sounds familiar. Didn't we just go through this about a year
ago?

Constant velocity and constant vacuum are both correct during cruising
operation with smaller butterfly openings. CV carbs are well-suited to
constant cruising at small throttle openings. That's what they are intended
to do and the "constant vacuum" or "constant velocity" description relates to
cruising operation.

If you keep the venturi area relatively constant, venturi velocity must
remain relatively constant and the engineers can meet their air pollution
targets.

Large throttle openings are something else...

It's still possible to defeat the designer's intent by raising the jet needle
too high so the carb comes onto the main jet prematurely and the engine has
to gargle the excess gasoline.

Even technical writers are confused by the CV carb's operation.

Suzuki describes the OEM CV carb on my GSXR as a "variable venturi type,
whose venturi area is increased or decreased automatically by the piston,
according to the vacuum present on the downstream side of the venturi.
Vacuum is admitted into the diaphragm chamber through an orifice provided in
the piston."

Actually, both statements are incorrect, obviously one cannot admit something
that does not exist except as a hypothetical construct to describe a
phenomenon caused by some other physical force, such as ambient air pressure.

The piston isn't lifted by "vacuum present downstream of the the venturi",
there is an AIR hole in the center of the piston, which is in the center of
the venturi and the engine sucks the AIR from above the diaphragm. AIR coming
in from under the diaphragm is then able to lift the slide.

Suzuki continues with the erroneous vacuum notion: "Rising vacuum overcomes
the spring force, causing the piston to rise to increase the said area and
thus prevent the air velocity from rising. Thus, air velocity is kept
relatively constant
for improved fuel atomization and for securing the optimum ratio of fuel to
air in the mixture."

Actually, if vacuum lifts the piston, any time the rider coasts downhill with
the throttle completely closed, the piston would fly to the top of its travel
and the engine would be running on the main jet!

During closed throttle coasting operation, vacuum increases, but less air is
pumped out of the chamber above the diaphragm. Air velocity and engine vacuum
are high enough to draw sufficient fuel out of the float bowl through the
tiny idle jet.

Again, an ignorant shade tree tuner can tweak the idle mixture screws too far
open and the exhaust will rumble and backfire as excess fuel burns off in the
pipes.

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