 |
|
|
Accelerator Pump Modifications |
|
 |
|
For a discussion of accelerator pump theory and normal tuning procedures,
see Tuning the Accelerator Pump in the maintenance
section of the FAQ. |
|
First, check the duration of
your accelerator pump (AP) squirt to see which mod you need.
Remove the air box boot off the carburetor, and wedge it behind the frame,
on the left side of the bike. Some prefer to remove the subframe and the
airbox completely for a more direct view.
Use a flashlight to look down the throat and you will see the brass jet
just behind the throttle plate on the bottom of the throat and slightly to
the right of the middle.
Crack the throttle with the engine off. Be sure the fuel bowl is full, and
that the pump diaphragm is loaded with gas. The best way to do this is to
ride the bike around the block before hand, cracking the throttle numerous
times to make sure it is well primed. A stream of gas should shoot out 4
to 8 feet. It will last between about a 1/2 second to 4 seconds.
Now time the duration of the squirt. You can either do this with a
stopwatch or use a camcorder to record the squirt. Then replay the squirt
frame by frame (30 frames/sec) and calculate the duration.
If you have a long AP squirt (3 – 4 seconds), you need the BK mod
If your AP squirt is too short (barely a dribble), you need the HB/DOC mod
and likely the BK mod as well.
|
(From TT member Motoman393, TT member Mark
Cantrell)
Similar to the way leak jets reduce the volume of the
AP squirt, the BK mod reduces the duration of the squirt. Most of these
bikes come with enough or too much and too long AP squirt. Larger leak
jets and the BK mod control each.
The BK mod came with a suggestion of 0.3 seconds or a
certain gap between the screw and the cam. This was suggested by Brian
Kinney (Tim Ferry’s mechanic). You may have noticed that a top 5 kind of
MX/SX rider like Ferry doesn't lug his bike around the track. He keeps the
RPMs up, always picks an appropriate gear, and doesn't ride in the woods
where the speed changes are more drastic. The 0.3 seconds may not be
appropriate for the average rider. My guess is somewhere between .5 and 1
seconds is about right, depending on the caliber of rider, the type of
track/woods, and if you keep it up off the bottom. At over maybe 6K RPMs,
the AP is just wasting fuel.
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. Most
agree that an AP squirt duration between 0.5 and 1 sec is optimum (see
Tuning the AP Squirt). This mod was developed for the 2001 models.
Many have suggested that this mod is not necessary on the 2002 models and
it is difficult to do on the 03 models (see below). It
is a good idea to measure your AP squirt duration before doing the mod.
Also, an aftermarket product called the P-38 will achieve the similar
results, however, at a higher price. Also, the P-38 is not adjustable.
Brian Kinney is Tim Ferry's factory mechanic (and the
originator of the BK mod) and the following were his instructions.
Click image to enlarge
Procedure:
-
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 rising. 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 Motoman393
 |
You don’t have to use a 4mm
bolt he used a 6-32 1" long flat head screwdriver bolt. He used a spring
out of a writing pen. His stock pump timing was around 4.5 secs and now it
is around .35 sec. |
|
|
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. |
|
|
The squirt of the gas was also
off on his bike...it should spray gas so that it just clears the
slide...but his sprayed when the slide was about halfway up the stroke.
|
|
|
The jetting he ended up with:
168 main, 42 pilot (1 3/4 turns out), needle in middle position.
|
|
(From TT member DOC)
This mod also allows you
to control the timing of the AP squirt. Some have found (especially on newer
bikes) that the AP squirt duration is actually too short. In which case, the BK
mod is of no help. This mod increases the duration of the AP squirt. In most
cases, the BK mod will be required afterwards to fine tune the duration.
Procedure
Refer to your owner’s service manual page 7 – 7 and read the discussion regarding the
leak jet.
The leak jet is a bypass for the AP jet (brass jet inside the carburetor throat).
When fuel leaves the AP some will be bypassed and released back into the fuel
bowl via the leak jet.
Take a flat head screw driver and remove the leak jet.
Block the leak jet with something, so all of the fuel is pumped through the AP jet.
The best way is to use a small ball bearing that just fits in the leak jet
housing. there is a small lip at the bottom of the housing, so this will stop the
bearing at the right height. If you use a ball bearing that is too small, in
might fall past the lip and block the fuel passage that leads to the AP jet.
TT member yzfmxer states that a 1/8" ball bearing is perfect
TT member vtss5000 found a bolt with the same thread
and simply cut a section the same length as the jet, and cut a small slot
into the end so i could use a blade screw driver to tighten it into place.
Many TT members solder the leak jet closed. Just make
sure to get the jet good and hot with the iron, and be sure to use flux.
Eventually the solder will flow into it. Then use a thin screwdriver to
hog out the screw slot so you can install it.
Replace the leak jet.
You will now have a squirt duration that is somewhere around 4 seconds.
To remove the ball bearing again, just remove the leak jet and pop it out. If it is a bit
tight, remove the acc. pump diaphragm assembly (seen on page 4 - 17) and use a
piece of wire to push it out again.
Four seconds is now way too long for an AP squirt duration. So now you need to do is
the BK mod (see above). This way you will have total control of the pump
duration.
If the bike is jetted correctly before doing this mod, you might find that the
jetting is too rich after the mod. Try one clip leaner on the needle, and down
two sizes on the main jet. The pilot may need minor adjustment.
You may also need to fine-tune your AP timing screw, because the strength of the
squirt will be much stronger now. It has a mark on it, so try moving it either
way until you can crack the throttle at idle without it stalling.
|
|
The leak jet changes still apply to the 2003 models. Apparently the 2003 carburetor was
redesigned and as a result there is no place to drill the hole for the BK mod.
As far as we know, no one has come up with a solution. At present, the leak jet
changes and the P-38 aftermarket pump cover appear to be the best solutions for
too long or a AP squirt duration. There is no current information
regarding the HB/Doc mod on the 2003 models. However, it is known that the
2003 models still have the leak jet, so assuming your AP squirt is too short
this mod should still apply. |
BK Mod for 2003 Models
(From DirtRider.net member Dan)
The hole must be drilled in the throttle cover between
the bottom mounting tab and the drain hose, there is not much room there
but you can clean out a nice spot with a dremel tool to make room for the
bolt. Drill the hole at a slight angle to get more surface of the bolt on
the cam. Used a 6-32 tap and a 1" bolt and washer and a spring that fits
the bolt. Then after you got it set to where you want it put some silicone
on it to keep it in place, and to seal anything that my have been a
problem around the threads. |
ThumperTalk Archives (updated
12/16/03)
|
|
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:
BK mod, bog, leak jet, acceleratop pump, AP, P-38. |
|
|
|
