that. You need to compare a hopped-up two-stroke against a hopped-up
four-stroke to get the answer you are
looking for.” Then Jamie said, “I’ll
modify the YZ250 two-stroke.” Wait,
Jamie took a beat-up 2007 YZ250
two-stroke, which is the same as the
2017 YZ250, save for the plastics, and
rebuilt it from scratch while adding
his secret sauce to the engine. Let the
2017 YAMAHA YZ250F
The Yamaha YZ250F Jamie built
for us was the crème de la crème.
No expense was spared. It was made
to go head to head against the factory bikes on the Supercross circuit.
Not possible? Jamie’s not worried.
He knows top-level teams hold their
engine development close to their
chests. This is where Jamie has a
niche market. He will give anyone
the best engine money can buy,
just as long as they can front the
cash. It comes with a steep price
tag, but Twisted Development has
enough engines on the AMA circuit
that Jamie can back up his claims.
The $7000 price tag for the engine
does not included the $1000 Hinson
clutch or $1050 FMF exhaust. When
it comes to building top-notch thumpers, it is all in the parts. Jamie has
his own spec on the piston and cams.
The crankshaft was rebuilt by Crank
Works. The transmission was sent out
for a treatment process to reduce friction, and the bearings were switched
out for ceramic.
Probably the coolest feature of the
bike is the dual-fuel injector system
that he borrowed from Kawasaki
to use on other brands. The second
injector is mounted upstream in the
airbox. Jamie didn’t have luck with a
second injector right off the bat, but
he knew the theory had potential. So,
he went out and bought more than 10
different injectors and spent months
testing them. Each injector had a
different spray pattern—from sharp
pin-like sprays to fine mist sprays.
The sweep of the sprays varied from
35 to 45 degrees. Jamie found the
potential he was looking for with a
25-degree spray pattern. This patten
filled the velocity stack and tricked it
into thinking the spray duration was
longer than it actually was.
The second injector is useless without the correct mapping. A Vortex
ignition is used in harmony with the
dual-fuel injectors. The front injector
is programmed to come on at 0- to
25-percent throttle. This injector handles the initial raw fuel to keep the
bike from bogging, coughing and stalling. After 25-percent throttle, there is
a linear transfer from the front injector
to the rear injector, which initially
kicks in at 33-percent throttle. Output
is based on throttle position and
rpm, so the engine can benefit from
the increased upstream atomization.
Bottom line: the raw droplets that
were being sprayed really close to
the throttle body now become a fog
of fuel, making the gas more volatile.
The atomization of the second injector