Tuning The Main Jet System - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

Section ENGINE PREPARATION
04

TUNING THE MAIN JET SYSTEM

, A01C4LS
Before operating the snowmobile, make sure all parts, including clutch and drive belt, are in good
operating condition.
1. Operate snowmobile at wide open throttle for several minutes on a flat, well packed surface.
Change main jet if snowmobile fails to achieve maximum rpm or labors at high rpm.
2. Continue to operate at wide open throttle and shut off ignition before releasing throttle. Examine
exhaust manifold and spark plugs to determine if fuel / air mixture is too lean.
o
NOTE : Do not change jet sizes by more than one increment (step) at a time.
3. If the exhaust manifold or spark plug insulator is dark brown or black, the fuel/ air mixture is too
rich. Decrease jet size.
4. If the exhaust manifold or spark plug insulator is very light in color, the fuel/ air mixture is too
lean. Increase jet size.
5. If you cannot determine the color, proceed as if fuel/ air mixture were too lean and increase jet
size. If operation improves, continue to increase jet size to obtain peak performance. If operation
becomes worse, decrease jet size to obtain peak performance.
6. After proper main jet is selected, recheck jet needle and needle jet.
o
NOTE : Rotax rotary valve twin cylinder engines with dual carburetors some times
require larger main jets on the magneto cylinder. Fuel particles are fed into the bottom of the
carburetors and tend to hug the bottom of the intake port. The rotary valve turns counterclockwise
(as viewed from the carburetors) and closes the
port from the top down. The fuel rich, bottom portion of flow is cut off first on the MAG side, while the
lean, top portion of flow is cut off first on the PTO side. This causes the MAG side cylinder to receive
less fuel so a richer main jet is installed to compensate. The amount of difference between main jets
will vary with engine design. High RPM engines with large carburetors and twin pipes will have more
stagger than will lower RPM engines with small carbs and a single pipe. (In some rare applications,
generally caused by the direction the single exhaust pipe curves, the PTO side maybe jetted slightly
richer than the mag side. )
04-76
8-10
20-22
100 Throttle valve opening
side port from the bottom up and the PTO side