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Its no big secret that the
clutch on the Yamaha thumpers is far from perfect. But there are many ways
to improve the performance or feel of the clutch with very little expense.
Detailed below are some of the more common clutch modifications discussed.
Also see: MXA: Bulletproof Clutch. |
79-Cent Fix for “Grabby” Clutch |
(TT Members McGrath and RandyWRF)
Many riders have described symptoms suggesting incomplete clutch pull. The symptoms include
not being able to start the bike in gear, the bike stalling when shifting from
neutral to 1st gear, and having difficulty finding neutral.
The issue with the stock hardware is that the spring CAVITY is not deep enough at full
clutch pull. What then happens is that the solid height of the spring (i.e.
fully compressed) is longer than the spring cavity (space between the pressure
plate and the large diameter washer under the retaining bolt) at full clutch
pull. So instead of the pressure plate traveling far enough to unload the
clutch plates it binds against the solid height of the spring. To prevent the
spring from compressing to its solid height we need to lengthen the cavity in which
it lives. To do this, spacers must be added to the clutch basket towers where
the bolts thread in. The spacers (washers) shim out the retaining bolts (one end
of the spring cavity) so that the springs play nice with the pressure plate
travel.
Procedure
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Remove the clutch cover
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Remove the 5 bolts for the springs
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Slip 6 mm (M6) washers over the bolts. Make sure they fit inside the springs. MAKE SURE the
thickness of the washers is equal! If one is thicker it will warp the pressure
plate and cause the same problem you're trying to fix! Measure the washers with
a micrometer or digital calipers. The thickness should be approximately 0.06 inches.
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Reassemble everything
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Readjust the clutch cable
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One important thing...MAKE SURE there are no grooves on the basket or the hub or you'll still have problems.
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YZF Clutch Springs in the WRF
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For unknown reasons, the
YZF clutch springs (see part numbers
are different than the WRF clutch springs, although the rest of the clutch
components appear to be the same. Many TTers, including myself, have indicated
that replacing the WRF springs with YZF springs improves the notchiness
and neutral problems on the WRF. The YZF springs are ~3/16" inch longer.
The paint on the WRF spring marking the tension strength is yellow and the
on the YZF spring is a pink. This notes a different tension. The pull is a
little stiffer with the YZF springs. |
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The 250F clutch has a tendency to get
more grabby as the clutch plates wear and also develops unpredictable clutch
take-up. A modification to increase lubrication of the clutch plates was recently written
up in Australasian Dirt Bike Magazine. I cannot seem to locate the article, but I have located a
graphic from that article.
You simply drill 15 more holes (5 sets of 3) in the inner
clutch hub (boss) to mirror the 15 already there. The holes are drilled in
every 3rd recess in the clutch hub. This effectively doubles the flow of
oil to the clutch plates. You shouldn't drill
more holes than recommended as you run the risk of starving the top-end of
lubrication and an oversupply of lubrication can lead to slippage.
The graphic does not indicate what size holes to drill. As there are
already holes there, I'd find a drill bit that would just fit inside one of the
existing holes.
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Removing Inner Plates (clutch boss spring and plate seat)
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(From TT member Polar_Bus)
"When my bike was
new, the shifting was terrible (notchy) and I could never find neutral (except in
panic, tearing up a steep hill). I also could not start my bike in gear. When
stopped with the bike in gear, and the clutch pulled I could feel my bike
wanting to creep forward. I ended up disassembling the clutch, and
re-assembling it without the two thin rings between the last friction plate, and
the basket. These rings transferred spinning drag from the basket to the
pressure plate, regardless of the clutch being engaged or not. After re-assembly
all of my problems were gone. My bike's transmission and clutch action are
perfect. I have approx 1700 mi, and have pulled the clutch apart after 900 mi,
and the clutch looked mint." Polar_Bus
Removing the clutch boss spring and plate seat and
replacing the last friction plate and clutch plate with
the standard thickness friction plate and clutch plate (see
part numbers
may also improve clutch performance. The only difference
between the inner and outer friction plates, and the rest of the friction
plates, is the surface area of the pads. I can only assume that this is because
the inner and outer plates mate up to the clutch boss and the pressure plate
which are made of aluminum.
There has been some concern expressed
that this "mod" may lead to premature clutch failure. Not many have actually
done this mod and there is no information regarding long-term results.
The cushion spring purpose is obviously to ensure that the
pressure is taken off the inner most friction plate when the clutch is
disengaged (lever in), while the seat plate is there to make sure the cushion
spring doesn't wear into the softer aluminum clutch boss. I'm just not sure why
this is needed, I can only think that maybe the inner most friction plate may
prematurely wear and/or overheat the mating surface of the clutch boss. A clue
may be whether aftermarket clutch bosses still use these rings or not.
There appears to be little
information and no conformation from other riders as to the effectiveness or
safety of this modification and I have not personally tried this one.
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Modifying the Shift Star Wheel
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(From TT member Ken Sherer)
This is not technically a clutch modification
but it does apparently help with the notchy shifting and accidental neutrals.
This modification may help with the issues surrounding the "neutral" problems
and "notchiness" on the 250Fs. This modification may help with the problem of
accidentally finding neutral, and may improve the notchiness. Basically when you
look at the ramps that you have to climb to shift from 1-2 and 2-1 its
immediately obvious why its a notchy transmission. Basically neutral is just too
damn big and easy to find. This is a death sentence on a motocross bike and a
constant source of annoyance for tight trail riders.
The Part
The shift star wheel is located under the right side cover under the
clutch. In order to remove it the clutch must come off. It costs about $35. The
ONLY tricky thing to remember here is that its not a 2-stroke so there are a few
o-rings and seals that connect the right side case to the engine. Without a
correct reassembly you may not have full oil pressure. Also, almost forgot, what
flops into each "slot" is a wheel held on the end of a spring loaded shaft. The
"wheel" is almost the diameter of a dime. This assembly is called a shift detent
mechanism.
Operating Theory
Note that there is a false neutral between 2-3,
3-4 and 4-5. All bikes have them because it is mechanically impossible for 2
gears to be engaged at once. If you get nothing else out of this remember that
it is there. And remember that when you are doing a big 4th gear jump,
especially when the gearsets are lightly loaded (ie; partial throttle) KEEP YOUR
FOOT AWAY FROM THE SHIFTER. Because there is always that slight chance that you
could unknowingly pull your tranny into one of these false neutrals, with just
the weight of your boot, and execute a perfect forward somersault. Not fun. The
tranny will work fine as long as you don't hambone it. That's why the false
neutrals are located on "points" on the star wheel. It is meant to flop into one
gear or the other. Do not interfere with this process when life and limb are at
stake. If you still have doubts put your bike on a stand. Snick it into 3rd. Now
rotate the back wheel back and forth as you ever so slowly pull the bike into
4th. At one point it will go into neutral. Now you know where you are: you are
balanced on one of the "points" on the shifter star wheel.
Procedure
Here is the stock part
with marks where is should be modified. I used a file to do it. Remember
that the point here is to make the ramps on either side of neutral similar to
the ramps for the other gears. You also want to reduce the size of neutral
itself. But keep the 2 "points" as sharp as you can as you can so the shift
detent wheel can't rest there.
Final notes and warnings:
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Its an easily reversible modification as a replace part is relatively cheap.
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Understand the process by shifting the bike
when its apart. Its important to get the "feel" of things so you know how far
you should proceed with the filing.
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Neutral will become harder to find, of course. This is the goal you want.
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There is a possibility that the shift detent
can "balance" on the points between 1-2 or 2-1 after you are done filling.
This could lead to some funny neutrals when the bike is idling by itself. I
have had ZERO problems, in fact that's why I started filing in the first
place. But be warned. I would not go revving the piss out of the engine (in
neutral / clutch engaged) with your friend standing in front of the bike.
- Two things we all get out of this
(regardless as to whether you do the mod or not):
- Watch the weight of your foot on the shifter when jumping.
- Don't stand in front of a bike that is
being revved to the moon with the clutch engaged. Neutral is a very, very
relative thing. It can come and go at the worst possible moments!
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Figure 1 |
Figure 2 |
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Figure 3 |
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Click images to enlarge |
Lengthening the Clutch Actuator |
Cut and Weld
Yamaha has tweaked the YZ426 to the nth degree since its inception five years ago,
but many riders, raised on two-strokes, find the clutch pull of a big
thumper a chore. A common modification on the big brother thumpers is to
lengthen the clutch actuator to lessen the clutch pull. This was a common
modification on older Maico's as well. Although this has traditionally
been described as a modification for the 400Fs and 426Fs, many have
successfully done it on the 250Fs, as well, with good results. You'll need access to a welder and a
modicum of mechanical skills to do this mod, but it's nothing that can't
be done in an afternoon. Here is a
link to a nice write-up scanned from a MotoCross Action Magazine article.
YZ400F Clutch Arm
Another method is to use the clutch actuator from a 1998-1999 YZ400F. This
part is a drop in replacement on the 250Fs and is ~5 mm longer. The
install is very easy and looks much better than the "modified" arm that
was illustrated in MXA.
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98-99 YZ400F
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WR/YZ250F
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| Total Lenth |
52 mm
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47 mm
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| Length from pivot point to tip |
42 mm
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37 mm
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Procedure:
- Drain the oil
- Remove the clutch cable
- Remove the clutch cover
- Remove the clutch pack
- Pull the clutch basket out about 1/2 of an inch
- Remove the bolt and retainer around the top of the clutch actuator
arm
- Slide the actuator arm out and install the replacement
- Position the actuator arm and slide the clutch basket back in
- Test the arm (while applying pressure against the clutch rod inside
the basket , move the actuator arm to make sure it is moving the rod.
- Reinstall the clutch pack and the clutch cover
- Fill with oil and reattach the clutch cable
- Adjust the cable
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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: clutch modifications,
washer fix, 79 cent, hard neutral, clutch mods. |
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