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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. Here is a link to a
template that can be cut out. |
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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. |
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Figure 1
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Figure 2
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Click on images to enlarge
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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. Click
here for more info.
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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
(Figure 12). However,
that will set you back $104.95.
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Update for 2003-2005 Model
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Yamaha
completely redesigned the airbox for the 2003 model with no tools for air filter
access (Figure 3). 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 to the 2003 WRF, 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 (Figure 4).
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 as the airbox is still quite restricted by the battery.
There are three ways to remove the snorkel. (Figures 5-7)
One is to use a
hammer and chisel (or screwdriver) to split the snorkel down
the middle. The pieces can then be pulled out through the
airbox.
Remove the
battery. Then use two flat blade screwdrivers on each side of
the battery strap bracket ant pry it out of the way. Have
another person pull the snorkel out with channel locks.
If you want to
be sure and save your snorkel:
Remove the
rivets from the battery strap clip
Remove the
battery strap clip
Slide the
snorkel out
Re-install
the battery strap clip with new rivets
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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. To download this template
click here. 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. Many will also cut out a similar shaped opening on
the bottom of the airbox and also place heat resistance screen
over the openings. (Figure 8)
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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?"
(Figures 13-15).
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.
TT member Junior_Vet
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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 (Figures
9-11). Images provided by Brad Robinson (TT member
brobixr250). |
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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 (Figure 16). |
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Figure 3 |
Figure 4 |
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Replacing with YZ125/250 Airbox
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| The airbox from the two-stroke models can be
mounted in the subframe. This does require use of the two-stroke
side panels. |
ThumperTalk Archives (updated 12/16/03)
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Listed here are what
I consider the most helpful threads. There are more threads on this topic in the ThumperTalk archives. To find them,
search the YZ250F/WR250F forum. Suggested keywords:
airbox, airbox lid, snorkel, 03 airbox, 2003 airbox free mods, free modifications. |
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