WHAT IS IT? Twisted Development’s YZ250F Spark
Plug Hold Down Clamp is a product that many Yamaha
YZ250F owners don’t know they need.
WHAT’S IT COST? $49.95.
CONTACT? www.td-racing.com or (337) 781-6400.
WHAT STANDS OUT? Here’s a list of things that stand
out with the Twisted Development YZ250F Spark Plug
Hold Down Clamp.
(1) Problem. Last year when the MXA wrecking crew
was running its then-new 2015 Yamaha YZ250F on the
dyno, we were disturbed by the lack of top-end power.
We pulled the fuel tank off to check the electric cables
from the dyno to the engine, and to our surprise we
discovered that the spark plug cap had popped up.
We pushed it back down, and the YZ250F ran fine on
the dyno and produced the kind of numbers that we
predicted—but only for three runs. Then it went south
again. Sure enough, we reached under the tank and
pushed the plug cap down with our fingers. It had
popped up again. We called Yamaha’s R&D department to
ask if they had ever heard of this problem. They said they
hadn’t, but suggested that we make sure the bleed hole
on the side of the cylinder was clear of debris. We had
already done that. To finish our dyno runs, we wadded up
an air filter and squeezed it between the top of the plug
cap and the bottom of the gas tank. That stopped the
plug cap from blowing up. Later, we pulled the gas tank
and safety-wired the spark plug cap down by bridging
across it. This was a cheap and simple fix.
( 2) The issue. In the past, when we have gotten water
down in the spark plug hole, the water has boiled when
the engine has gotten hot and the expanding steam blows
the spark plug cap off. That could be a problem on the
YZ250F, but as long as the cylinder bleed hole that exits
just behind the exhaust pipe on the right side of the
cylinder is clear, there is no way for water to get past
Yamaha’s full-coverage, plug-on-style, spark plug cap,
which leads us to believe that the heat of the cylinder
expands the air, minus the steam, to pop the cap loose.
( 3) Recognition. We’ve never had a test rider come
in and say that he thought the spark plug cap was
loose. Instead, test riders complain about hitting the rev
limiter too soon, the engine cutting out at high speed or
a lack of power when wide open. As soon as they say
that, we check the spark plug cap (it can be reached by
fingertip from the left side of the engine). Why doesn’t
the engine stop running? Because the spark can still
jump the gap between the cap and plug; however,
because the spark’s arc gets shorter at high rpm, it
becomes hit-and-miss at 11,000 rpm.
( 4) Factory teams. We asked Star Yamaha’s Steve
Lamson if he had this problem on Martin’s and Webb’s
YZ250Fs. Steve said that they made an aluminum
holder to cross from one valve cover bolt to the top
of the plug cap. That is when we discovered the
Twisted Development Spark Plug Hold Down Clamp. It
is a very fancy version of the factory Yamaha part but
works on the same principle. It keeps constant pressure
on the spark plug cap to prevent it from
popping off during long motos. Not surprisingly, Twisted
Development is owned by former AMA National
mechanic Jamie Ellis.
( 5) Performance. It works great.
WHAT’S THE SQUAWK? We were happy with our
safety-wire solution, but it had to be rewired every time
we wanted to check the plug. The Twisted part swings
out of the way by loosening a valve cover bolt.
If you race a 2014 or 2015 Yamaha YZ250F,
you need this—even if you don’t know it.
TWISTED DEVELOPMENT YZ250F SPARK PLUG CLAMP
MXA TEAM TESTED