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The stock
airbox on the WR250F does not allow adequate airflow, which robs
power from the bike. The YZ250F does not have a lid for the airbox
and therefore does not have this limitation. At a minimum, the
snorkel should be removed. Most 2001-2002 WRF riders completely
remove the entire airbox lid and place it in permanent storage or
the landfill (Figure 1). The airbox lid is there for your protection
so when removed you must be more careful when washing the bike or
riding in extreme conditions.
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Supplied with the WRF
is a piece of rubber that mounts between the seat and tank that
will help deflect water and mud out of the airbox. If you don't
have this piece one can be made easily enough. |
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I modified the airbox
lid by removing the snorkel and placing a screen over the opening.
I then installed 8 Uni air filter vents in the front portion. When
riding in extreme conditions, I place the modified cover on the
airbox for added protection (Figure 2). |
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Cut a couple of layers
off some pantyhose and duct tape this over the top of the air box
as sort of a pre-screen. It breathes just fine and the vibration
from the bike will knock the chunks of sand, mud, pebbles off.
Keeps your air filter cleaner also. |
Figure 1
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Figure 2
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Others have gone one step
further and trimmed to top ridge of the air box flush with the sides
of the airbox. This mod also works on the YZ250F. This not only
makes it easier to remove and install the air filter it even adds
additional airflow for potentially greater performance gains.
Modifying the Air Filter Cage
Cutting out the Screen
Want to know something that all the old timers
know, but that the factories don't want you to know? Sure you do!
The wire screen inside a four-stroke's air filter cage is not
conducive to producing horsepower. In the good old days, four-stroke
riders cut the wire screen out of the cage to get maximum air flow
to the top end. So why don't the factories remove the wire screens to get more power? Because
of the danger of a backfire. Four-stroke engines can, and do, belch
flame back through the carburetor on occasion. It is possible for this
backfire to light the air filter on fire. Thus, every four-stroke
manufacturer puts a wire screen in the airbox to stop the flame.
At your own risk, you can cut the screen out of your air filter
cage to allow more air flow. Use tin snips and try to trim the wire
screen as close as possible to the edge of the cage. Be careful, the
remnants of the wire screen are sharp. Also, the
bike will run just a tad leaner without the screen because airflow
is increased.
YZ125/250 Air Filter Cage
The air filter cage for the YZ125 does not have a backfire screen
(because it's a two-stroke). According to Larry Roesseler, the YZ125/250 cage will fit the YZ/WR250F
airbox. Apparently you use the cage from the same model year. Again, do this at your own risk of a backfire setting the
filter on fire.
Twin Air Power Filter/CageTwin air now makes four-stroke
specific air filter that is flame resistant (and allows you to remove the restrictive wire screen from
your air box). This filter can be used with either of the above
solutions to increase air flow with little risks of the filter
catching fire. It costs $25.95. Twin Air has developed
this filter with
a laminated coating that is flame resistant. When the engine belches
fire, the laminate keeps the filter foam from lighting up. Very
simple and very cool. The only noticeable difference between a
Back-Fire filter and a regular Twin Air filter is that the laminated
foam is black instead of cream colored. You install the Back-Fire
filter just as you would the stocker. Twin Air also makes a trick
works-like billet aluminum air cage that does not have a screen. However,
that will set you back $104.95.
Update for 2003/2004 Model
Yamaha
completely redesigned the airbox for the 2003 WR250F with no tools for air filter
access. Three D-ring quick-release fasteners remove the side cover and
one wire band releases the air filter. The airbox lid has a pocket
with a rubber damper for battery placement. With the addition of the
electric starter, intake airflow has been reduced due to the battery
under the seat. While these features are beneficial to cleaning the
air filter they make modifications to the airbox more difficult.
Trimming the rim of the airbox is not necessary on the 2003 YZ250F
(see above) as the airbox has been redesigned.
The safest place to get
more intake air on the WRF is the side panel next to the air
filter. Before you grab a hole saw and start drilling you need to take
a few things into consideration. Without any type of shield you are
asking for trouble as debris will be sucked inside clogging the air
filter. You also stand a chance of ruining the side cover.
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The snorkel can be easily removed to improve airflow but the performance gain is minimal.
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Australian dealers supply
2003 WRF buyers with a template from Yamaha. This template shows
locations to cut out on the right side of the airbox (under the
side panel) to allow more airflow. The 2004 WRF has the areas
for these cutouts already etched into the airbox. Simply cut out
the plastic along the etchings with a sharp knife.
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In order to
significantly open up the top of the airbox, the battery must be relocated. Most TT members
relocate the battery behind the number plate or mount it on the rear fender (via a fender bag).
Details are sketchy but will be added when they are available. Then the top is cut out of the airbox.
Listed below are a few ideas from ThumperTalk. As far as I know
none have been widely accepted or tested.
TT Member DH247
First you remove the entire top portion of the airbox and
replace it with a fine wire mesh that covers the top of the box
to prevent larger particulate from entering. It will be folded
under on the edges and stapled to the box (on the outside for
maximum protection).
Measure the appropriate location of the tray (almost directly
above it's current location) and mark this out on the underside
of your seat. Cut the appropriate size hole in the hard plastic
of the seat.
Next cannibalize the holding tray to include the 4 sides, and
the hooks for the rubber strap. Achieve the desired depth from
inserting the tray into the bottom of the seat (upside down of
course!). mark this and attach 6 small L-brackets to secure the
box into place. Use screws on the seat portion and bolts on the
box with round heads (so the battery can still slide in and
out). The battery can be mounted upside down as the top is on
it's side already.
Install a snap on connector (and some extra wire to get
everything maneuvered when you take the seat off) for the
positive and negative wires. The connector is kind of like the
ones from battery powered RC cars. Plug and play kind of deal.
Put the battery in the seat and strap it in. For added
reassurance add 2 swivel pieces that you can rotate over the
battery (like the back of a picture frame but stronger). Put the
seat on the bike close enough to hook up the connectors to the
battery and then attach the seat. Done deal.
The final result is that you don't have to move the battery far,
and it still opens up the airbox dramatically.
From TT member Barton
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"My idea (which I also haven't done yet) is to
build a metal box to mount under the rear fender. Lined with thin,
dense foam similar to what is already around the battery, it
should protect the battery from rocks, etc. What I haven't
measured is it's effect on wheel travel. I don't want to bottom
out and rip the thing off of the fender. A "backing plate",
installed on the top of the fender, could help hold the box on
more firmly, but it might tear the fender off if the wheel hits
it.
There is also a LITTLE room below the air box, in front of the
wheel, just above the swing arm, but that looks risky too. I
wonder if a different profile battery is available from the
manufacturer with similar voltage and amp-hour characteristics?"
TT member ButchWR250F
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Created a solution for the overly restrictive
03 WR250 air box lid. He crafted a battery pack out of SLA-Type
batterries, removed the airbox lid, and crafted a mount for the
battery. The new battery weighs about 20 oz (with mount), and is
tucked up inside the airbox out of the way. It takes up about the
same amount of room as the old air holes in the old lid, leaving
almost 3 times the airflow into the filter. He stated that he
could get 7-8 starts with a 12V 1.3Ah battery without having to
recharge and that the battery would rapidly recharge while riding.
He got the battery from zbattery.com (model
number UB1213). It was only $7.50 + shipping. See images below.
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One can also replace the WRF airbox with a stock YZF airbox. This option is costly
and the battery must still be relocated. Also the YZF side panels
must also be used. |
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Uni air filter vents like the ones shown above can be installed but will also have minimal
performance gains compared to opening the top of the box. |
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Barnum Pro Products air
box mod is made of billet aluminum with fine mesh screens that
allow more airflow yet keep small particles out of the air box.
You do need to clean the air filter more often but the power
increase is well worth it. Performance wise the $68.95 kit is
inexpensive horsepower. |
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