Hac High Altitude Compensator - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

THEORY
The high altitude compensator is a mechanical device designed to vary the pressure in the float
bowl chamber relative to air density. Air density is affected by variations in elevation and air
temperature. As the elevation goes up from sea level, the air density decreases and as temperatures
increase air density also decreases. When going down in elevation, air density increases and as
temperatures get lower, air density also increases. The H.A.C. increases or decreases the amount
of air pressure in the float bowl, thus changing the fuel flow into the carburetor venturi. The unit is
connected to the carburetor via several passages, which control the atmospheric pressure in the
float bowl chamber. As a snowmobile goes up in altitude without a H. A. C., the air density
decreases, but the same amount of fuel is delivered to the engine. The amount of oxygen available
to the engine is lower, so we have a vehicle that runs rich. The H.A.C. is designed to lower the
pressure in the float bowl chamber at higher altitudes and increase the pressure at lower elevations.
The unit is lightweight and requires no battery or separate control device.
The fuel delivery rate of the carburetor depends on the jet sizes and on the pressure acting on the
fuel. This pressure results from the pressure difference between float chamber and fuel exit in the
carburetor venturi (needle jet). Pressure increase in the float chamber leads to richer mixture,
pressure decrease to leaner mixture. This effect is utilized in the Automatic High Altitude
Compensator (H.A.C.).
The necessary pressure reduction in the float chamber for mixture leaning is taken from the
venturi depression. This low pressure is guided via connection 1 into a pressure attenuator
consisting of the variable jets DI and D2. By the air flow through the jets DI and D2 the pressure is
reduced to a certain ratio and fed into the float chamber via connection 2. The connection 3 leads
to the atmosphere via a vent tube.
The air in the sealed diaphragm chamber 6 expands more or less, depending on the air density and
displaces via a diaphragm 7 the profiled corrector needle 8 in the jet bores DI and D2.
With decreasing air density the jet passage area of D2 increases and the jet passage area of DI
decreases. In consequence the pressure in the carburetor float chamber decreases and the fuel/air
mixture gets leaner.
The sealed chamber 6 is filled with dry air. Moisture in the chamber can cause freezing which
would lead to an incorrect mixture. For this reason, no adjustments to the H.A.C. are recommended.
The screw 5 is sealed and should not be tampered with. If the H.A.C. is out of adjustment, damaged
or tampered with, a new H.A.C. unit should be installed.
Section ENGINE PREPARATION
04
04-89