VOL. 53 ISSUE 28 JULY 19, 2016 P87
The mid-pipe diameter is
larger and the bend radius has
been increased, further boosting
mid-to-top power. These chang-
es are all aimed at increasing the
power from 7000 to 14,000 rpm
without compromising bottom-
end grunt.
With increased power for
2017, Yamaha added a heat
treatment to the engine cases
and the connecting rod has
a new shape with a material
change for improved durability.
Revisions to the ECU make the
rev limiter sequence quicker
and more abruptly, letting the
YZ250F engine pull all the way
to the 14,000 rpm limiter. Last
year's power was decreased
as it approached the 14,000
rpm rev limiter on a longer cut
sequence.
Inside the cases, third and
fifth gears are beefed up and oil
supply holes and grooves have
been added to the transmission
shafts. To improve shifts, the
shift drum has been updated
and the shifter lever is a new
material for a 76% increase in
strength. Yamaha claims the
stiffer shifter makes the distance
between gears feel shorter. The
position of the shift lever is also
changed to match the now 5mm
ENGINE TWEAKS AND
IMPROVEMENTS
Wanting to improve overall pow-
er to the already fast YZ250F
rearward slanted engine, Yama-
ha gave it a new cylinder head
with an even straighter intake
port and new shape, increasing
the flow rate. Larger valve seats
also increase flow. In conjunc-
tion with the new cylinder, the
air boot was shortened and
a new throttle body joint now
features an aluminum core for
improved fuel vaporization. New
intake and exhaust cams have
more lift and stronger valve
springs.
It doesn't look much
different, but Yamaha went
through the 2017 YZ250F
with a fine-tooth comb.
(Right) Sharp bends have
been taken out of the exhaust pipe
routing to improve flow. (Far right)
New engine mounts have been
redesigned to improve feel. It's
been discovered lately that engine
mounts do a lot more than just
hold the engine in place.
