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Know a little something about maintenance, fixing, tuning, or modifying MX, offroad, & dual sport motorcycles, ATV or UTV? Or, maybe you have mad skills riding or racing them? Whatever the case, if you have valuable knowledge & experiences that relates to motorcycles, ATVs, or UTVs, please help your fellow riders by sharing your best tips, tricks, and how to articles.

    Way Fast Whitey
    *this is for the TT'ers without a bearing press*
    if your rear/front wheel have play in them chances are your bearings in the hub are worn. i've used this method with great results.
    1. remove the wheel from swingarm(for rear wheel)
    2. get the old bearings out, a punch should work, just work around the bearing til it comes free.
    3. go to your local hardware store, and buy a bolt long enough to go through the hub, 2 washers for the size of the bolt/nut, and 2 washers about the size of the new bearings(must be smaller then bearing, but the colser the size the better)
    4.with the new bearings ready for installation, and the inside of the hub cleaned, you can start your new bearings inside the hub.
    5.with the new bearings started in the right direction,put 1 small washer on the bolt, then follow with 1 large washer, fit this through the hub.
    6. once the bolt is through the hub, put the other big washer on the bolt, then the small washer, then the nut, finger tighten.
    7. get your apporiately sized deep socket on the nut and tighten, checking regularly to be sure the bearings are going in right.
    8. once the bearings are seated, you can release the bolt and remove, reinstall on swingarm and do the same to the front.
    good luck
    Trailryder42
    Hey all,
    I recently had a 440 kit installed in my "00XR400 and with the heavy duty clutch springs that were part of the kit, the clutch work was a little tougher than I like. So I bought a Hebo hydraulic universal clutch kit from my buddy Jim Cook at Smackover Motorsports. The reviews of it I'd read said the pull was slightly easier and longer than the Magura and quality was as good or better.
    Thing is, after looking around all over the web, I could find no help or suggestions on where to mount the slave cylinder on an XR. These are 3 links I used to help me, tho none of them work specifically with the XR.
    http://www.john-stichnoth.com/Hebo.html
    http://dirtrider.net/drn_tested/erider/hydrualic_clutch.html
    /forum/showthread.php?t=2151
    So after spending a few days experimenting with different potential mounting points, keeping in mind I need to take into consideration the amount of heat the unit will see and keep it accessable yet somewhat protected, I put it on the left side of the forward main frame downtube. I fashioned a bracket that would use the left mounting point for the fuel tank. Having an IMS 4 gal tank made options scarce. Maybe if anyone else wants to do this project on an XR, between the links provided and what I'll add, it will be a little easier.
    I used the long cable and sheath provided in the kit, without cutting/altering it's length. And the stock style cable keeper/adjuster that uses the stock bracket mounted to the right case cover. The end of the cable is bare so you use the cable clamp provided in the kit that fits the XRs throw lever.

    Here's a picture of the slave cylinder mount. I used a temporary spacer setup the same thickness as the fueltank mount for initial fitting of the cylinder. I wrapped the cylinder in the same heat wrap I used on the header to help shield it from any heat. I used plastic spiral wrap on the braided line to add some abrasion and heat resistance to it.

    Here's final installation pictures.
    [
    My decomp lever would no longer fit on the handlebars, so for now, until I figure out something better, I mounted it to the bark buster.

    Here's a tip I discovered. The bleeder nipple/fitting bolt threads are a sloppy fit. When trying to bleed the system with my vacuum bleed tool, air/vacuum escapes around the threads of the nipple and prevents the tool from working properly. I had to remove the nipple and wrap its threads with teflon tape. The system bled fine after that. Getting the amount of lever pull I needed was tricky and took me awhile to figure out. Took the bike for a spin up and down the street to check it and Man is this thing sweet! I bet it cut lever pull difficulty by 50% or more.
    The braided line is longer than I needed for this install and figuring out a way to route the excess and get the end clocked in the orientation I needed it was a pain. Later I might have it shortend.
    Hope this might help someone else that is contemplating this mod.
    Trailryder42
    mark112
    A lot of people loosen the brackets that attach your clutch or brake assembly to the handle bar, so in case of an accident the assembly will spin rather than break. But its difficult to get the right tension and the friction factor changes with water, mud, etc. which can cause the assembly to work its way loose and move while riding.
    I find the best idea is to wrap Teflon tape (plumbers tape) around the handle bar where the the clutch and brake assembly will be mounted. The Teflon is frictionless, so you can still tighten your clamps firm but it allows the assembly to spin with force in the event of a crash, but not rattle loose while riding.
    250Thumpher
    Whenever I would change my oil, I'd clean the oil filter as well (why not, it's a SS oil filter!)
    But every time I would loosen all the bolts, oil would just pour out of it, onto my water pump, onto my frame, then on my skidplate, then on the stand, then on the floor, then down the driveway, then down the dr... Ok I went tooo far with that, but you get the point...
    So after trying to find a way to make it as clean as possible, aluminum foil did the trick. Just tear off a big piece and fold it in half (hot dog) then in 3's (hamburger). Then just mold it into place and make a U so the majority of the oil goes onto your funnel and into the pan.
    Even if you do not have a perfect seal, a lot of the oil will flow onto the "funnel" and if some oil doesn't go down it, at least you won't have a huge mess.
    A little tape on the corners helps out too. It keeps the foil in place while you are removing the oil filter cover. You may have to play with how big of a piece you need, but just use whatever it takes to get the job done. Trick it to get it thick that you can mold it and it will stay, but not too thick that the oil will not run on it.
    BannerUp

    By BannerUp, in Articles,

    Race sag is the distance the suspension sags under the combined weight of the bike and rider from its fully extended (topped out, no load) position. If your race sag is correct, your suspension is in the middle of its range, where it can handle the widest range of riding conditions without topping or bottoming out. For most riders, a race sag of 90-100 mm translates to the correct preload for dynamic conditions.
    Static sag is the distance the suspension sags under the bikes weight alone, without a rider, from its fully extended position. Once you get your race sag correct, the static sag will tell you whether or not you have the correct spring for your riding weight. So always check static sag after setting your race sag, because the preload adjustment affects both.
    STEP 1 => Put your bike on a stand, and measure the vertical distance from the rear axle to a spot on the rear fender. Record this value as M0…

    STEP 2 => Take your bike off the stand, put on your riding gear, take a standing position, and measure again. Record this value as M1…

    STEP 3 => Subtract M1 from M0… this is your race sag. If it’s between 90 and 100mm, skip to Step 5. If not, put your bike on a stand, lubricate the threads on the body of the shock, and proceed to Step 4.
    STEP 4 => If your race sag in Step 3 was significantly less than 90 mm, decrease the preload by moving the rings up the shock body (ccw). If it’s significantly more than 100 mm, increase preload by moving rings down shock body (cw)… Repeat Steps 2 through 4 until race sag is between 90 and 100mm, then continue with Step 5.

    STEP 5 => With your race sag now correct, and your bike off the stand, measure again, but this time with the bike under its own weight. Record this value as M2…

    STEP 6 => Subtract M2 from M0… this is your static sag. If it’s between 25 and 35mm, your preload and spring rate are correct. Take your bike out for a test ride, then come back to this forum, and do “Adjust Your Dampening.” If your static sag is not between 25 and 35mm, proceed to Step 7…
    STEP 7 => If your static sag is less than 25 mm with the correct race sag, your spring is probably too soft for your riding weight. What happened is this: to get your race sag correct, you set the preload higher than it would have been with the correct (stiffer) spring. So the bike sags less than the recommended value under its own weight. If your static sag is more than 35mm with the correct race sag, your spring is probably too stiff for your riding weight. What happened is this: to get your race sag correct, you set the preload lower than it would have been with the correct (softer) spring. So the bike sags more than the recommended value under its own weight. In either case, go to www.racetech.com, and checkout the recommended spring rates for your bike and riding weight. Buy the spring and install it, then recheck your race and static sag, and adjust as necessary. When you’re done, get in touch -- we’ll go for a ride…
    BannerUp
    You’ll be better able to adjust your suspension correctly if you first understand how it was designed to operate and can identify symptoms of incorrect operation. So let’s take a look at some of the…
    BASIC PRINCIPLES
    Preload and spring rate primarily affect how your suspension handles the big stuff, whereas dampening primarily affects how it handles the small stuff. But there is considerable overlap in who does what, so it’s a team effort. When they work together properly, you get both comfort and control over a wide range of riding conditions.
    Compression dampening works with the spring to resist the wheel’s upward movement during a bump. In both the front and the rear suspension, for example, it helps the spring resist bottoming on big bumps, sharp rocks or deep whoops.
    Rebound dampening works against the spring to resist the wheel’s downward movement after a bump. In the shock, for example, it keeps the rear spring from jamming the seat into your butt, and in the forks, keeps the front springs from pushing the bars into your face.
    If they have not been adjusted to work together, the wheels bounce and slide rather than roll. And you get a lousy ride. Or crash and burn in the boonies. Too much compression dampening, for example, helps the spring too much, which produces a jarring, haphazard and uncomfortable ride over even the smallest rocks and bumps.
    Too little compression dampening can also give you a “hard” ride if you have a “soft” spring -- especially if there is too much rebound dampening. The soft spring, and less-than-ideal compression dampening, allow the wheel to come up too much when it hits a bump, and the excessive rebound dampening keeps the wheel from returning to its “normal” position in time for the next bump. After a series of bumps, the suspension gets "stuck in a squat” with maybe an inch or two of travel.
    This is packing, and shows its ugly face as harshness in the handlebars or side-to-side swapping of the rear wheel. Even with the correct spring.
    Rebound dampening in the forks plays a major role in how well your bike corners. The compression of the springs during a turn “push” the wheel into the ground. The correct rebound dampening “holds” the spring’s rate of return so this “push” is maintained until the turn is completed. You front wheel develops good “cone effect” and your bike tracks through the turn smoothly and accurately.
    Too little rebound dampening allows this “push” to get weak, then go away before the turn is complete. The wheel turns late or loses traction, and your bike turns wide or washes out. Too much dampening allows this “push” to be stronger and longer than necessary to complete the turn. The front wheel bites too deeply, and your bike turns early and inside.
    Compared to your present settings, more dampening slows the wheel’s movement for a firmer ride, whereas less dampening speeds the wheel’s movement for a softer ride.
    For both rebound and compression dampening, turn the clickers out (CCW) to decrease dampening and in (CW) to increase dampening.

    ***********


    Even the most commonly accepted principles and rules of thumb should be tested in the real world of trial and error, revision and adjustment, so let’s take a…
    TEST RIDE
    Shock Rebound
    Ride through rocks, roots or bumps leading into and out of a corner. If the rear wheel hops when braking for the corner or accelerating out of it, soften RD a few clicks and try again.
    Ride over a log, ledge or rock. If the rear kicks up badly, stiffen RD a few clicks.
    Ride through some deep whoops on hard ground. If your bike doesn’t track straight and the rear wheel doesn’t extend to the bottom of each whoop before the next one, or swaps badly, soften RD and try again. If that doesn’t help, stiffen CD a few clicks, and try again.
    Shock Compression
    Ride a trail with small rocks, roots and bumps. If the rear end feels harsh and bouncy, soften the CD. If it wallows, you’re riding the spring -- add CD.
    Ride off your biggest jump. If your shock bottoms badly, add two clicks of CD. If it still bottoms badly, and your bike is not equipped with a Hi-Speed adjuster, try a stiffer spring.
    Fork Rebound
    Ride a short, sweeping turn. If your bike resists the turn, understeers, drifts to the outside, or the tire loses steering traction and washes out, add RD. If it oversteers, turns too quickly to the inside, or the tire bites too hard and knifes into the ground, decrease RD.
    Fork Compression
    Ride a trail with small rocks, roots and bumps. If the handlebars feel harsh, soften the CD. If not, add CD until they do feel harsh, then back off a click or two.
    Adjustment Tips
    Adjust the rider sag and check bike sag to ensure you’ve got the correct spring.
    Check the pressure in both tires, and bleed the air pressure in both forks.
    Use as little dampening as possible to get a “plush” ride, not a “soft” ride.
    Small or smooth bumps want more compression dampening than big or sharp ones.
    Hard trails want less dampening, and soft trails want more dampening
    But sand requires very stiff settings – don’t be surprised if you max CW your clickers.
    Use the Hi-Speed Adjuster for jumps, and the standard CD adjuster for bumps.
    Improper riding technique can fool you into misadjusting your suspension.
    You can make the wheel hop if you lock the brake or pull the clutch in a turn.
    You can make your bike over or under steer with incorrect body position.

    ************


    In the life of every problem, there is a time when it’s big enough to see but small enough to solve. So let’s take a look at a few solutions to some of the most common…
    HANDLING PROBLEMS
    Jarring => harsh ride in the handlebars on relatively small but sharp bumps
    Decrease CD & RD so wheel comes up and goes down faster
    Try a fork spring with a lower spring rate for a softer ride
    Packing => harsh ride in handlebars on larger bumps
    Increase CD in the forks to limit the compression
    Decrease RD to help the wheel return more quickly
    Try stiffer fork springs
    Headshake => front wheel ocsillates side to side, especially in soft stuff
    Move forks lower in triple clamps to increase rake and straight-line stability
    Decrease RD in forks so wheel rolls over rather than ploughs into ground
    Try stiffer front springs and softer rear spring to move center of gravity rearward
    Oversteers => bike turns too quickly, goes inside the turn, front wheel knifes
    Move forks lower in the triple clamps to increase rake, slow down turning
    Decrease RD on both forks to slow down the turning
    Install stiffer fork spring so wheel won’t sink in and bite too much
    Understeers => bike turns too slowly, drifts wide -- front wheel pushes, washes out
    Move forks higher in triple clamps to decrease rake and speed up turning
    Increase RD on both forks to increase “cone effect” and speed up turning
    Bleed air pressure in both forks to stop the front wheel from pushing
    Try softer fork springs so wheel bites rather than pushes the ground
    Kickup => rear wheel hops straight up, seat pops you in the butt
    Increase RD to slow down wheel’s return after hitting a bump
    Swapping => rear wheel hops side to side in hard to loamy whoops
    Increase CD, and/or decrease RD to eliminate packing
    Braking Hop => rear wheel hops excessively when braking for bumps or a corner
    Decrease shock’s RD to help wheel follow the bumps more quickly
    Acceleration Hop => rear wheel hops badly on rough ground during acceleration
    Decrease RD to help wheel follow the bumps more quickly
    If that doesn’t lessen the kickup, decrease the shock’s CD
    Acceleration Spin => the rear wheel loses traction under heavy acceleration
    Increase RD to improve rear wheel “squat”
    Guest

    By Guest, in Articles,

    I wrote this when I was a member of another board. My guide was written for sportbikes, but it all applies. I'll submit this for consideration as a 'sticky' if the board admin wants to use it.
    --
    Replacing and/or bleeding brake lines. Nothing short of pulling a bank of carburetors causes the average sportbiker more distress.
    I for one believe braided lines are one of the best bang-for-the-buck mods you can do for your motorcycle. Maybe the very best mod a guy can do for a around a hundred bucks. You get better braking feel and increased stopping power because the lines don't expand under pressure like the cheap rubber OEM ones do. And trust me on this, doing the work is not nearly the black art you might think it is.
    Now that you've decided to make the leap, go ahead and buy your line kits from wherever you like. There are some great site sponsor e-vendors right here on Thumpertalk who can help you out, I'm sure.
    You can get your fluid and Speedbleeders from them too (I highly recommend the latter purchase - you'll need three and they're ten bucks apiece, well worth the cash).
    As for the whole debate on what brake fluid to use, guys get excited about synthetic DOT 5.1; the selling point on this stuff is that it resists boiling better than DOT 4 and is less "hygroscopic" too - which means it tends to absorb less moisture over time than DOT 4.
    However, unless you're talking about a dedicated track bike, I don't see the value. It's typically more costly than plain-jane DOT 4 and cannot be mixed with anything else if you're low on fluid and in a pinch.
    Fresh DOT 4, changed out once a year in the spring, will serve you just fine and will cost you less. Thus, I suggest you simply order a bottle of Galfer Super DOT 4 (or any other name brand) when you order your lines and Speedbleeders from the vendor of your choice.
    I would also recommend the purchase of a basic Mityvac Brake Bleeding Kit, the cheap plastic one that sells lots of places for $25 or $30. (For some reason, CycleBrakes.com is about $15 higher than they should be on this tool.)
    Don't get intimidated by the relatively lengthy instruction manual that comes with the Mityvac. In fact, you just as well toss it in the trash. This tool is a simple vacuum pump, nothing tricky about it. Affix the clear vinyl tubing to the pump with the inline capture cup attached and you're ready to go.
    Let's start with the front brake lines, shall we? 🙂
    Put your bike on front and rear work stands, or a rear stand at least. You want it level and sitting on a solid, stable platform.
    Remove your windscreen and set it aside. It's easier to work on the front of the bike w/o the screen on, and this way you avoid mucking it up with a spilled drip of brake fluid (more on that later). Put some old towels on top of your instrument cluster, fairing, and fuel tank too.
    You should have some plastic clip-type fasteners which hold the factory lines to the back of the fender. Remove those so the midsection of the lines are free.
    The first thing you'll use the pump for is sucking out the bulk fluid from the reservoir cup. Unscrew and remove the plastic cover, insert the free end of the vinyl tubing, and pump to your heart's content. Easy as pie.
    Next you'll want to drape some more towels on the bottom half of your front wheel because you will still have fluid in the line, and when you unbolt it from the caliper, it's going to dribble some. (Note: at the risk of insult, the hex-head fasteners which attach the lines to the master cylinder and calipers are called banjo bolts, so that's what I'll refer to them as from here on out.) Just have a jar or bottle handy so you can put the leaking free end of the stock rubber line inside to catch the majority of the draining fluid.
    A word on working with brake fluid: don't flip out if you get a drop on your fork, caliper, wheel or other painted surface. Stay calm, wipe it off with a dry paper towel (more absorbent than a cloth towel) and you'll be fine. You might want to clean the now dry spot with another paper towel and some Honda Spray Cleaner & Polish (to this day, I love Honda chems!) if you're really worried about it. The point is, people talk about brake fluid as if it's nitroglycerin or something sure to ruin your bike the instant it touches anything painted. Not true. Be careful, wipe up spills, and you'll be fine.
    Okay, once the majority of the fluid in the line has drained, unbolt the banjo at the master cylinder up top on the right clip-on. Have a paper towel ready to capture any drips from the top of the line. Carefully thread the line down through the fairing and out the bottom of the bike. You'll be tossing it, the OEM banjo bolts, and all used crush washers (new ones come with your Galfer kit).
    Get the new lines ready. The front kit is a two line affair with a long and a short line. Take a look at the back of the package - it'll tell you where to place the new crush washers, how many to use, etc. (basically at each and every 'joint'). The short line is the right side which will go against the master; the left line rides on top of it. Bolt everything up, make sure the lines aren't binding on anything at full lock, then torque to 12 to 15 ft-lbs.
    Unscrew and remove the stock bleeder nipples from both the right and left calipers. Again, you may get a drip. Just have a paper towel handy, this is not the end of the world. Replace the stock bleeders with the new Speedbleeders you bought. No need to seal the threads with Teflon tape or wheel bearing grease as the SB's come with thread sealant already applied. Tighten them down with a small wrench, but don't overtighten.
    Now you're going to attach the Mityvac to the left caliper's SB via the clear vinyl tubing. Set it down and get your bottle of DOT 4. Carefully pour it into the reservoir cup, almost all the way to the top (just leave enough room that it doesn't spill if you jiggle the bike a bit). Unscrew the SB 1/4 to 1/2 a turn. Begin pumping the Mityvac. You'll see fluid begin to disappear from the cup. Now you're beginning to prime the lines.
    At first you'll be sucking only air and you'll have to stop pumping from time to time so you can stand up and pour more DOT 4 into the cup (you don't want to let it get sucked dry because that means you've drawn air into the system and you'll be starting over).
    Eventually (it really doesn't take too long), you'll see fresh brake fluid coming out of the SB, through the clear tubing, and into the catch cup attached to the Mityvac. Keep pumping and pouring until you get mostly clean fluid coming through without big gaps of air pockets (small bubbles will remain, but don't fret, we'll take care of them in a few minutes).
    By now you might want to unscrew the Mity's catch cup and dump the fluid into another container. Snug up the SB on the left side. Go around the bike and prime the right line in the same manner described above. Once you get mostly clean fluid coming through, you've got both front lines primed and you're ready for the final bleed.
    At this point, you're done with the pump. You can remove it, leaving yourself a length of clear tubing to attach to the SB's and the catch cup at the end. I like to go back to the left side caliper and start the final bleed there because it's the longest line, but I have no scientific reason to support why I do this. I think you could begin on either side and have it work out just fine.
    Again, you'll want to back out your SB about a 1/4 to 1/2 turn. Attach your tubing and catch cup. Make sure you've got sufficient fluid inside the res. Give the brake lever a full squeeze, slow and firm, all the way to the bar. Then release, keeping an eye on your fluid level to make sure you don't run dry as you go. Repeat above as necessary.
    You should see movement in the fluid and a gradual reduction in not only the number of bubbles coming out the SB, but also in the size of them as well.
    If you've done things correctly, you should be pushing virtually unblemished, virgin DOT 4 through the lines and out the SB in very short order, just a number of pulls on the lever.
    You might need your husband, wife, S.O. or roomie here at the end, because I like to close the SB about mid-stroke on the brake lever just to make sure the line fluid is under pressure when the escape route is sealed off. (You can simply tighten the SB after your last pull of the lever and be okay, however. The internal check-valve is what prevents air getting into the system.)
    Now go over to the other side of the bike and repeat the final bleed process.
    Once you're done up front, you pretty much follow the same steps to replace, prime, and bleed the rear line.
    The final thing I like to do is use a nylon tie to hold back the front brake lever with the bike on its side stand and bars locked to the left. This places the master cylinder as high as you can get it. Take the plastic handle end of a screwdriver and gently rap the calipers, banjo bolts, and lines from top to bottom. By the next morning, any teeny-tiny bubbles which might have evaded your expert bleeding adventures should have, in theory, escaped up and into the res.
    You're all done! Go inside, shower up, and crack a beer. You've earned it!

    Note: you can do all this without Speedbleeders and a Mityvac, but the process changes a bit. I'd have to type up another walk-thru if that's the route you go, so I implore you to pony up the extra coin to get the SB's and pump!
    😏
    Ride well, all.
    Sarge
    KeithBoyd
    After tightening up nuts or bolts, dab a small amount of Tipp-ex on the bolt/nut head and onto the bracket/frame/fitting surrounding. This way, or you can look at them while on a ride or during a break and see if they`ve slackened off (marks not lining up). The Tipp-ex will scrape off or wash off after a couple of rides and will not be as permanent as paint or a Sharpie pen.
    Rick_Kienle
    Four stroke carburetors have accelerator pumps that produce a squirt only while the throttle is being added, not at a constant throttle or trailing throttle. The purpose is to make up the fuel lost to low vacuum until the revs build and recreate the vacuum. Like any other carburetor circuit (e.g., pilot circuit, needle circuit, or main jet circuit), there may be more fuel added or less fuel added than is needed. In addition, the AP squirt may be the right amount but not last long enough (to build revs) or may be too long (being rich after revs build).
     
    The amount and duration of the AP squirt is dependent on a few things:
    AP timing linkage. This controls when the squirt starts. Leak jet size. This controls the volume of the AP squirt. How quickly you twist the throttle. With a slow roll of the throttle, the AP is not activated. Length of stub on the AP diaphragm. This controls the duration of the AP squirt.

    When you twist the throttle, the AP linkage pushes a rod down against the AP diaphragm which pushes gas through a passage going to the carburetor venture. The squirt of gas compensates for the big gulp of air the carburetor sucked in. The sudden drop in vacuum causes less fuel to be sucked through the normal jets. To help in fine tuning this squirt, there are adjustments. The leak jet is like a bleed hole in the AP squirt passage. A slow twist of the throttle will push a small amount of gas through the leak jet (which is the path of least resistance) and almost none will make it through the whole passage into the venturi. If you twist the throttle quicker, it tries to force more fuel through the passage which can't all go through the leak jet, so the rest flows through the whole passage and squirts into the venturi. The timing screw (on the external linkage) lets you time when the squirt starts and to some extent the duration, i.e. earlier squirt equals slightly longer duration. The length of the rivet on the diaphragm will also control the duration. A longer rivet will cause the diaphragm to bottom out sooner and limit the travel of the diaphragm, therefore reducing the duration of the squirt. A shorter rivet will allow longer travel and therefore a longer squirt duration.
     


     
    The figure is a schematic sketch of the accelerator pump (AP) circuit. When the bike is running with the AP fuel reservoir full, and you whack the throttle, the actuator rod (green) gets depressed. This pushes the AP diaphragm (blue) down forcing fuel out the bottom passage. As you can see, the passage allows the gas to go in one of two directions. One passage leads directly to the AP nozzle in the carburetor venturi. The other passage leads to the Leak Jet (red) and back into the carburetor bowl.
     
    A couple of things become immediately apparent. One is that by varying the size of the leak jet, we can vary the amount of fuel coming back to the bowl and, therefore the amount that goes to the AP nozzle. A larger leak jet allows more fuel back to the bowl, and less fuel to the AP nozzle. A smaller leak jet allows less fuel back to the bowl, and more to the AP nozzle. A such, a larger leak jet leads to shorter squirt duration/volume and a smaller leak jet leads longer squirt duration/volume.
     
    Also, the rivet on the bottom of the AP diaphragm limits its travel. A longer rivet would equate to shorter squirt duration. Length of the AP diaphragm is measured, measured from the top of the AP diaphragm to the bottom of the rivet. Stock diaphragm length is 7.5 mm.
     
    It is also important to set AP timing screw (on the left side of the carburetor, under the cover). If you set it too tightly, it partially depresses the actuator rod. This holds the AP diaphragm down, and reduces the volume of fuel in the AP fuel reservoir leading to shorter squirt duration/volume. If the AP timing screw is set too loose, the AP squirt lags the movement of the carburetor slide, also potentially producing a bog.
     
    When the AP squirt is not correct in volume and duration, the bike will stumble and potentially stall with a quick blip of the throttle off of idle. Searching the list, and this site, I found that several other people had the same problem. Also, there appears to be no one fix that works on all bikes.
     
    Especially on the 03 and later models (on which the BK mod is difficult or impossible), it is likely that the AP can be tuned pretty close using different leak jets, AP diaphragms, and adjusting the timing linkage (see below).
     
    Approach to Curing the Bog
     
    The big issue is that the same fix does not work across the board. There is no "quick" fix. This is because like all jetting, the AP function is dependent on multiple factors such as (but to a lesser degree than other jetting circuits):
    Riding style Temperature and elevation The other jetting circuits (primarily the pilot circuit but some on initial needle taper as well). Any mods/upgrades you have done, primarily those that affect intake or air flow such as carb mods, airbox mods, cam mods, porting, and exhaust.

    So the only option you have is to meticulously work through the process until you get satisfactory results. Here are several things you can check and/or adjust that will affect AP function and the degree of off-idle bog you experience:
    Pilot circuit jetting. You must have your pilot jet, pilot air jet, and fuel screw all dialed correctly. One note here for WRF owners, the YZF has a larger pilot air jet, at least on the older models. I have a YZF pilot air jet in my WRF because I am running YZ exhaust cam timing, have opened my air box and have an aftermarket exhaust. Also dialing in your PJ and fuel screw settings with a tachometer is extremely beneficial to the process. Details can be found at https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/jetting.htm#PJ Needle taper and clip position Idle speed, make sure your idle speed is correct. I run mine toward the high end or recommended range ~1900 rpm. Again, set this with a tachometer. AP linkage timing. The AP squirt needs to clear the raising of the slide. This can be done by visual adjustment. There are also a few ways of tuning this. One is in the manual, the other was reported in one of the magazines. Both methods are detailed at https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap.htm Leak jet size. Again one size does not fit all. The correct size needs to be determined by timing your AP squirt and adjusting appropriately. AP diaphragm stud length. Again one size does not fit all. Also determined by timing the AP squirt. This is used in combination with the leak jet to further adjust squirt time. Very few have needed to make changes to the AP diaphragm. But keep in mine, this part wears out. So on older bikes it may need to be replaced. PowerNow. The powernow has some affect on the bog but is not a complete fix. P-38 Lightning. This is a bolt-on accelerator pump plate that supposedly cures the bog while adding horsepower and response. This product will also ease in starting. This product is basically a bolt on replacement for the stock AP cover on the bottom of the carburetor. You do have to use the OEM O-rings and screws. This item has a stud in the bottom of the chamber that limits travel of the AP diaphragm. There are also minor changes to the fuel ports. The same results can be achieved by using different AP diaphragms with different stud length. Some minor jetting changes may be required with this mod. My bike only required adjustment of the fuel screw. This did not completely cure my bog. Boyesen AP Cover. This is a new product that I just saw in the most recent issue of MXA. It is also a bolt on replacement AP cover but it uses a different approach than the P-38. This item moves the fuel passages to different locations within the reservoir. I do not have any information regarding the effectiveness of this part. If anyone has real world experience with this part please let us know. BK Mod. This is essentially a mod that allows control over the timing and volume of the AP squirt. This mod is easily applied to the 2001-2002 models and was the factory racing teams first approach to fixing the bog. It can be done on 2003+ models but it is slightly more difficult. The same results can be achieved with leak jet and diaphragm changes. The only real advantage to the BK mod is that it is more adjustable and can even be adjusted "on the fly." See https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap_mods.htm The HB/Doc mod. This is essentially blocking the leak jet completely. If the #35 leak jet still produces too short a squirt, then blocking the leak jet may be necessary. See https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap_mods.htm
    Also, please keep in mind this CANNOT be tested on the stand or with the bike in neutral. Even a properly tuned bike will stall or cough if the throttle is quickly twisted from closed to WOT if there is no load on the motor. The off-idle response MUST BE TESTED while riding under normal conditions.

    Now, I clearly understand that all of this feels daunting to those who are uncomfortable with jetting and for those who have never delved into their accelerator pump. But it is really not rocket science once you start digging into it. The goal is to achieve an AP squirt of 0.5-1.0 seconds that just misses the slide. Also keep in mind, that for most it is quite easy to cure 90% of the bog with:
    Tuning the pilot and needle circuit Setting idle speed properly Adjusting the AP timing linkage Leak jet changes

    Everything else listed above is for those who need that extra 10%.
     

    Timing the AP Squirt Duration
     
    Another source of frustration seems to be the process of accurately timing the AP squirt. This is quite essential to fixing your bog. This is best done by digital video but can be accomplished with analog video or with a stopwatch. Here is how to do it, step by step:
    Make sure the float 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 cracking the throttle numerous times to make sure it is well primed. When I am doing multiple measurements or testing different settings. I take the carb off the bike and place it in a vice. I then use a funnel to keep the float bowl full of gas.
    Remove seat, tank (but make sure carb float bowl is full of gas), rear fender, airbox/boot and subframe. If you have a powernow, it is easier to remove that as well. Use a flashlight to peer into the carb intake. You will see the slide in the closed position. Just in front of the slide (toward you) and just to the left of center is a small brass nipple that sticks up. This is your AP squirt passage. If you quickly twist the throttle you will see the slide move up rapidly and a stream of gas will emerge from the AP nozzle toward the motor. This is your AP squirt. Obtain the help of a friend or family member. My son is a good AP timing helper. Have your helper hold the flashlight on the carb intake and twist the throttle. Hold a video camera or digital camera with video functions at an appropriate distance to allow visualization of the AP squirt as well as enough light to adequately see. Observe the AP squirt "visually" and have a stop watch or stopwatch function on a wrist watch.
    [*]Have your helper quickly twist the throttle form zero to WOT as fast as possible and hold it open for a few seconds. Either record the squirt or time it with the stopwatch. [*]Record or time multiple squirts (4-5) and average the results [*]Calculating the AP squirt
    With a stopwatch, simply record the time and average the results. If recorded with video go to you TV or computer (depending on video or analog). You must have the ability to review the tape in frame by frame mode. You must also know what the frame rate of the recorder is. Most analog and digital video cameras record at 30 frames/second. Some digital cameras with a video function record at 15 frames/second and some newer digital recorders record >30 frames/second. Playback your recordings frame by frame. Find the frame in which the first appearance of the squirt is recorded. This is frame 1. Go through the recording frame by frame counting the number of frames the squirt is visible. Record this number for each of your recordings. The calculation is simple # frames divided by #frames/second. For example, if your squirt lasted 18 frames and you were recording at 30 frames/second then the calculation would be 18/30 = a squirt time of 0.6 seconds. So this for each of the recordings and average your results.



    Once you know what your current AP squirt time, you can determine which direction and by how much you need to adjust.
     

    Leak Jet
     
    Newer bikes (either 2001 and up or just 250Fs) have a leak jet that leak's some of the squirt back into the bowl. As indicated above, the leak jet gives you the flexibility to adjust the pump beyond the limits of those without it. The AP is purposely built too strong so a smaller leak jet would send most of the fuel into the venturi and a larger leak jet would send less into the venturi (that is, more would leak back into the bowl). This allows adjustment from too much to too little (volume). Part numbers for available leak jets can be found in the Yamaha Part Numbers section. The leak jets are numbered according to the size of the hole. For example, a #90 has a 0.90 mm diameter opening, a #80 has a 0.80 mm opening, and so on.
     
    There was a Yamaha service bulletin in 2001 regarding the use of leak jets and AP diaphragms. The date of the Yamaha service bulletin is 8/24/01 and it is labeled: "Report Number: 01-002" "Models - YZ250~426F, WR250F~426F(All Years): Subject - Optional Accelerator Pump Diaphragms and Leak Jets." Most service departments should have it in a book on the shelf somewhere for those who are interested. There are two other service bulletins available in the links below.
     
    Procedure
    Remove the rear fender, subframe, airbox, and airboot. I also take the bell of the carburetor. The bell is not removable on the 2003-2005 models. Adjust your idle speed to that recommended in the manual. Adjust AP timing linkage (per manual) Measure your AP squirt. Drop the bowl on the carburetor (you don't have to pull the carburetor). In the bottom of the bowl, about 1/2 way from the center to the back, brake side is a tiny brass jet with a flat head screwdriver slot in it. Unscrew it and see what number it has on it. Bigger number jets give less AP squirt (more is wasted back into bowl). Smaller number jets give more AP squirt (less is wasted back into bowl). The goal in adjusting the AP is to select enough squirt to get it past the low RPMs but not enough to outlast the low RPMs or create a too rich condition during the low RPMs. It will burble like a two stroke with the choke on if too much and just have low acceleration. It will cough, cut out, or die without bucking at all if too little. Experiment with different leak jet sizes, using the charts below as a starting point, until the bike runs the best and your AP squirt duration is in the desired range (~0.5-1 seconds, some say 0.4-0.8 seconds).

    AP Squirt Times with Various Leak Jets (using the 2003-2013 WR/YZ 250F's standard diaphragm - 5JG-14940-76-00)
     

    Leak Jet Duration (sec)
    #90 0.25
    #75 0.267
    #70 0.50
    #55 0.67-0.75
    #50 0.467
    #40 0.667
    #35 0.8 - 1.13
    Closed 1.4 - 1.733
     
    Additional Observations
    The spray nozzle in the venturi has a diameter of 0.3 mm which is significantly smaller than the leak jets in most cases. This makes leak jets very effective for setting your accelerator pump duration. Do all of your testing on the bike, not on the stand. Warm it up by riding for at least 15 minutes before you judge the effectiveness of any changes. If you are unable to get your squirt duration in the proper range, you might consider changing out the AP diaphragm (see below). If you want to further tune your squirt duration, say somewhere between where the #50 and #60 leak jet puts you, try this. Install the #50 (longer squirt of the two), and back off the AP timing screw in ¼ turn increments, until you get exactly the squirt you want. This slightly depresses the actuator rod, lessening the gas volume, decreasing the squirt duration. Don’t forget to have the rest of your jetting and carburetor adjustments in good shape. Especially the float level, pilot jetting, and main jet/needle combinations.

    Yamaha OEM Part Numbers
     

    YZF/WRF Leak Jet: 4JT-1494F-XX-00 (for all model years)
     
    #135 XX = 34
    #130 XX = 33
    #125 XX = 32
    #120 XX = 31
    #115 XX = 30
    #110 XX = 29
    #105 XX = 28
    #100 XX = 27
    #95 XX = 25 (05 WR250F standard)
    #90 XX = 23 (04-05 YZ250F standard)
    #85 XX = 21
    #80 XX = 19 (06-08 YZ250F standard
    #70 XX = 15 (03-04, 06-09, 11-13 WR250F, 03 WR450F, 09-11 YZ250F standard)
    #60 XX = 11 (01-02 WR250F, 04, 07-11 WR450F standard)
    #55 XX = 09 (06-09 YZ450F standard)
    #50 XX = 07 (05-06 WR450F standard)
    #45 XX = 05
    #40 XX = 03
    #35 XX = 01
     
    For finer tweaking, Sudco has Keihin leak jets from #35 to #140 in increments of 2-3.
     
    Keihin part number: N424-52-xxx
     
    Jet Size Sudco Part no.
    #70 019.764
    #68 019.763
    #65 019.762
    #62 019.761
    #60 019.760
    #58 019.759
    #55 019.758
    #52 019.757
    #50 019.756
    #48 019.755
    #45 019.754
    #42 019.753
    #40 019.752
    #38 019.751
    #35 019.750
     
    AP Diaphragm
     
    There are also four different AP diaphragms available from Yamaha. Each diaphragm has a different length stub (rivet) on the bottom to bottom out on the pump cover. The measurement is from the top, where the rod contacts, to the bottom end of the rivet. Part numbers are located in the Yamaha Part Numbers section. Larger numbers will reduce the AP squirt duration by limiting the travel of the AP diaphragm, similar to the P-38. There was a revision to the 2002 model's upper dish to make them 1mm taller. This makes the rivet start closer to the bottom initially for a shorter stroke. This is likely why the BK mod does not seem to be necessary on 2002+ models.
     
    On a YZ426F carburetor, the 5JG-14940-19-00 diaphragm squirts for about 1.3 seconds with closed leak jet. This is nearly 1/2 the flow of the -76-00 (standard on 250Fs), so all the above durations could be cut in half using the 5JG-14940-19-00 (9.0mm) diaphragm.
     
    Additionally, the 2008 Honda CRF450R introduced a new diaphragm with a much shorter rivet (~4.83mm) & a longer rod. Along with the shorter diaphragm rivet, Honda introduced an updated accelerator pump cover that moves the check valve from the float bowl to the accelerator pump cover. Read more details at CRFsOnly.com
     
    AP squirt Duration using OEM cover and #35 Leak Jet
    Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
    5JG-14940-18-00 / 8.00 / 0.80 - 0.93 / DPH #25 (WR 450F standard)
    5JG-14940-76-00 / 7.46 / 1.13 - 1.27 / DPH #30 (YZ/WR 250F standard)
    5JG-14940-17-00 / 7.01 / 1.33 / DPH #35 (YZ 450F standard)
    5JG-14940-16-00 / 5.96 / 1.47 / DPH #45
     
    AP squirt Duration using OEM cover and blocked Leak Jet
    Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
    5JG-14940-19-00 / 9.00 / 1.30 / DPH #15 (YZ 426F standard)
    5JG-14940-18-00 / 8.00 / 1.40 / DPH #25 (WR 450F standard)
    5JG-14940-76-00 / 7.46 / 2.07 / DPH #30 (YZ/WR 250F standard)
    5JG-14940-17-00 / 7.01 / 3.03 / DPH #35 (YZ 450F standard)
    5JG-14940-16-00 / 5.96 / 3.30 / DPH #45
     
    AP squirt Duration using P-38 and #35 Leak Jet
    Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
    5JG-14940-76-00 / 7.46 / 0.72 - 0.77 / DPH #30 (YZ/WR 250F standard)
     
    Accelerator Pump Timing Adjustment
     
    The other part that is often ignored is the adjustment screw that comes on the linkage stock. This is to set the starting point of the accelerator pump. Turning it in delays the start and turning it out advances the pump action to start earlier. An immediate squirt from idle would be zero delay from the "touch point", where the rod just touches the diaphragm. The linkage needs to be set with enough delay to keep the AP squirt from hitting the slide. Otherwise a "lean" bog would occur. There have been several methods discussed regarding setting this adjustment. Some say, simply turning the screw 1/2-1 turns out (from the all-the-way in position) is adequate. The OEM manual describes, in detail, the recommended method for setting the linkage. I do recommend visual inspection of the squirt and fine adjustment of this setting after using the manuals method. For unknown reasons, there are different "throttle valve heights" listed for the WRF and YZF.
     
    Throttle valve height
    YZ250F/YZ450F = 1.25 mm / 0.049 in WR250F = 1.5 mm / 0.059 in WR450F = 3.1 mm / 0.122 in

    Procedure (refer to the images below)
    In order for the throttle valve height (a) to achieve the specified value, tuck under the throttle valve plate (1) the rod (2) or other suitable spacer with the proper outer diameter. (Note: the diameter of the spacer or rod should equal the throttle valve height listed above. I use a hex wrench of the proper measurement and make sure the flat sides are aligned correctly to give the proper height.) Fully turn in the accelerator pump adjusting screw Check that the link lever (4) has free play (b ) by pushing lightly on it Gradually turn out the adjusting screw while moving the link lever until it has no more free play


     

    Accelerator Pump Modifications
     
    O-Ring Mod & Alternatives
    The O-ring mod involves adding a #78 o-ring between the accelerator pump linkage & the throttle linkage. This will make the squirt stronger at the expense of a slightly shorter squirt. The disadvantage to the o-ring is that over time, the o-ring will stretch out. Some JD Jet kits will include 2 o-rings (a thick and thin one).
     
    The second option is to wire tire the linkage together. While the wire tie won't wear out, you risk binding at WOT and breaking the plastic cam.

     
    The third option is a stiffer accelerator pump spring. They won't wear out and they won't bind, but they are the more expensive than safety wire or an o-ring.

     
    Three companies make such springs:
    Merge Racing #00-018 (potentially discontinued) (Installation instructions) R&D Racing Tokyo Mods #3901

     

    The following modifications were initially used before tuning with leak jets and AP diaphragms became common (generally prior to 2003). Although these mods may still apply in certain situations, they are generally not necessary if you have tuned your AP squirt with the methods previously described. However, one advantage to the BK mod is that it is adjustable "on-the-fly."
     
    BK Mod
     

     
    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).
     
    Brian Kinney is Tim Ferry's factory mechanic (and the originator of the BK mod) and the following were his instructions.
     
    Procedure (2001-2002 models):
    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.

    Procedure (2003+ models):
     

    The 2003 carburetor was redesigned and as a result there is no place to drill the hole for the BK mod. 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.
     

     
    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.

    HB/Doc Mod
     

    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
    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. TT members have found several ways to do this. Use a small ball bearing that just fits in the leak jet housing. 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. Four seconds is now way too long for an AP squirt duration. So now you need to do the BK mod. 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.
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