Notes Regarding Engine Modification - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

NOTES REGARDING ENGINE MODIFICATION

1. Tunnel porting
This procedure refers to the grinding out of the crankcase from the rotary valve inlet towards the
transfer ports at the cylinder base. The effort here streamlines the flow from the rotary valve
inlet to the cylinders. This modification benefits engines running at high RPM (8000 and up).
When installing larger carburetors, opening of the rotary valve cover and the crankcase openings
may also be included in a tunnel porting job to match the new carburetor bore. When installing
carburetors larger than 42 mm, however, do not enlarge the opening at the valve side of the
cover beyond 42 mm. Taper the opening smoothly from the carburetor flange down to 42 mm
on the disc valve side. The opening
towards the transfer ports.
Tunnel porting should be done on
2. Porting
When porting cylinders, remove
passages or poti-windows, but do not alter the outlet angle of any transfer ports. The ports and
their passages should be left smooth and clean. The only port worth spending time polishing is
the exhaust port.
If you are changing any port dimensions, be sure to chamfer all edges of the port windows when
you are finished.
Section
in the crankcase should match it at 42 mm and "trumpet" out
y by accomplished engine modifiers.
any burrs, rough spots or irregularities you may find in
ENGINE PREPARATION
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