(reposted)
First I
wish to say that this mod is entirely due to Brian Kinney, Garrett Berg
is the
first to have detailed and posted the mod to the Thumpertalk forum. The Second image Describes the GB/BKmod
in
conjunction with the Accelerator timing screw. More of both these will
be
explained below.
I am merely trying to explain the details on proper setup after the
mod. Thanks
Brian Kinney and Thanks Garrett Motoman393 Berg. Also note, the pics
are from
Garrett's site as well. I enhanced them for the settings detail.
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Taken from Garrett's Web Site
http://motoman393.thumpertalk.com
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Info:
This accel pump mod is ONLY for the
00'-02' YZ426,
01'-02' YZ/WR250F and 00'-02' WR400/426) this WILL NOT work on 98-99
YZ400's
and 99 WR400's. When this mod is done correctly is will make your bike
run and
start better. It also will give quicker throttle response and eliminate
the bog
off the bottom. Your jetting will most likely need to be richened up
since this
mod shortens the duration of gas sprayed (from 1.7 secs stock to .3
secs)
The Procedure that Brian Kinney gave me:
Brian Kinney is Tim Ferry's Factory mechanic (and the originator of the
BK mod)
and these were his instructions...To limit the accel
pump spray length you must drill and tap the pump cam stop and install
a 4mm
screw/spring combo. That will contact the pump cam. A hex socket bolt
works
well approx. 25mm long. Remove the subframe
so you
can look down the throat of the carb and with a stopwatch time the
length of
the pump spray when you stab the throttle. I usually click the
stopwatch at
exactly the same time I turn the throttle and click it again when I see
it
stop. It may seem weird but it works! Also you need to adjust the pump
timing
screw so that it does not hit the slide when it is raising. The timing
is close
when the spray just misses the slide. Then set the duration of the
spray to .3
seconds with the adjustment screw you just installed. This may sound
complicated but is the only way to get rid of the pesky bog off the
bottom.
Helpful Tips from me:
You don't have to use a 4mm bolt I used a 6-32 1" long flat head
screwdriver bolt. I also used a spring out of a pen that you write
with. My
stock pump timing was around 4.5 secs and now it is around .35 secs
(maybe that
was why is flooded easy and was sometimes hard to start when hot?) Also
when
drilling put a piece of metal (like a hacksaw blade) in-between where
the drill
bit will poke through and the black pump thingamajig LOL. The squirt of
the gas
was also off on my bike...it should spray gas so that it just clears
the
slide...but mine sprayed when the slide was about halfway up the
stroke!
Just by starting up the bike I could literally "feel" the difference,
it felt like the snap of a 250 2 stroke and didn't have any bog
whatsoever! You
will really be amazed by what $4 does to the performance of this bike!
The
jetting I ended up with: 168 main, 42 pilot (1 3/4 turns out), needle
in middle
position.
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The below image shows the Mods main component. This is the duration
screw added
to lesson the duration of the fuel Squirt
The 3rd Image is the side view of the complete mod and how it will work.
The last 4th Image is a side shot looking somewhat down the back side
of the
carb. I will now explain the setup.
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So to begin, if you have not done the mod yet follow the mod completely
as Motoman's instructions specify in the
first part of this
post above.
One side Note Very important. When drilling the 4mm hole into the
Carb,
place a thin piece of metal (Hack Saw blade) between the exit and the
plastic
cam.
**************************************************
In pic 1 I used a 4mm hex bolt, much easier to adjust.
Just set the bolt into place at first at the edge of the hole or just
leave it
out until the timing is adjusted.
Now for the timing
In pic 2 it shows the pump screw, it is important only to adjust this
clock or
counter clock wise in 1/8 turn increments. It is very sensitive and if
you pump
is working correctly you will see the spray change effectively.
Note Make sure the float bowl has Gas in it for this part, and don't
do this
near a flame
Take a look at pic 4, but you should be looking down the back side of
the carb,
keeping the carb as level as possible.
Turn the cam on the carb, you will notice a little brass nipple in the
right
corner just in front of the slide. This is the squirt nozzle.
If you twist the cam you will see a stream of fuel hit the slide as it
opens.
THE OBJECT IS TO HAVE THE SQUIRT JUST START AS THE SLIDE LEAVES ITS
BASE
so with that in mind if you turn the Cam and you see the squirt
immediately hit
the slide and the slide is not moving, you need to turn counter clock
wise the
Pump Time Screw 1/8th turn.
You should see an immediate effect on when the squirt starts.
Remember the squirt must start just as the Slide begins to open
Once you have the time done, it may take a few sets to get it correct.
You can
now work on duration. As you noticed the duration is long and the
squirt has
plenty of fuel.
So now comes the duration
I don't understand how the .3 or .5 duration stated, I am not that fast
to hold
a stop watch in one hand eyeball the spray and turn the cam at the same
time. I
am sorry but logic tells me unless you are a Cyborg,
its impossible.
Anyway the way I did this is to visually adjust the duration by sight
and feel.
The 426 only needs enough gas to get rid of the bog, on the other hand
not
enough and you still have the bog (Fuel Starvation).
The Distance between in Pic 2 on my sled is about 1/8 of an inch. I set
this by
watching the squirt as I opened the cam up. I tried a couple of diff
settings
but found that all you need is a split second of fuel to get the beast
to
breath fire at bottom.
So that is it except that you may have to adjust it for you area of
ride,
altitude, humidity, and weather will effect the GB/BKmod.
It does take some time but to adjust and get correct is a big payoff on
the
426.
I used the Hex Bolt because once I was able to get the Mod complete and
the
bike back together a simple quick adjust to fine tune in the woods was
perfect.
Again, it is a great mod, it can have dramatic effect on your ride if
done
correctly.
Hope it helps