KDX200 conversion for MX
The KDX comes lightly suspended for use in the woods so that the wheels will easily go over abrupt obstacles such as rocks and tree roots. It is not set up for handling jump landings on a motocross track. For that the damping must be more resistive to wheel movement or you will be bottoming out hard on landings. If you made large adjustments to the compression on the front and the compression/rebound on the rear then you will be able to handle the jumps but the ride will be stiff and will wear out your hands/arms rapidly. Kawasaki recommends 10W oil up front and 5W oil in the rear. I have found that (without revalving) for MX you need 15W up front (Bel Ray), and 7.5W in the shock. (mix 10W and 5W Bel Ray HVI Racing Suspension Fluid, $17 each from MX South). Otherwise you'll have to revalve, so its really the quickest/cheapest option. Also the fork oil level will have to be raised 10-15mm which will add to bottoming resistance on big jumps (due to more resistance from the compressed air that will be pressurized more than normal). If you have conventional forks (not upside-down) then you might also consider installing (just put under the fork spring and drill some holes in the aluminum tube) fork cartridge emulators for $170 from RaceTech They help the forks to feel softer in initial travel which helps since conventional forks can feel harsh once they are set up to not bottom harshly on landing from jumps. Click here to read about the difference between cartridge forks and conventional forks. You can put the shock heads 21mm diameter shim at the 7th shim position (counting from the big shim side) on both the rebound and compression side. That will produce more "give" with small bumps to compensate for stiffening of the valving with the thicker oil. On my bike that change was essential because before I did that the teflon head seal was being blown out of place on the rebound stroke (probably after bottoming out on jumps). I eventually replaced the shock with an Ohlins shock from a KTM enduro bike. I had to revalve it stiffer and change the dog bones on the linkages to keep the tire from hitting the fender but I really loved the smoothness of that shock. I think the Ohlins from a 2002 GasGas 200 EC has a length closer to that of the stock KDX shock.
Springs: Strangely enough the KDX comes with a front fork spring (.36kg/mm) thats correct for someone really lightweight, while the rear spring is for someone 190lbs or more. Available from MX Tech are fork springs (.4-.46, in increments of .02, for $135) and shock springs (4.0-5.2, in increments of .2, for $120). Here's a spring rate chart for intermediate MX raxcers:
rider fork shock
----- ---- -----
145lb .40kg/mm 4.4kg/mm
160 .41 4.5
175 .42 4.6
190 .43 4.7
Piston skirt mod
Shock linkage "dog bones": These come with the correct length for a steeper-than-normal front fork angle which gives better sharp turning ability for use in the woods but less high speed stability. That characteristic is enhanced by a shorter-than-normal wheelbase (which also increases sharp turning ability). So unless those are changed for longer ones, the handling will be "busy" on the MX track with a tendency to loose straight line stability which may result in swapping ends and/or doing a tank slapper. For the latter years of the KDX, longer dog bones are available from aftermarket suppliers, but not for the older models. I have an '89 model and I just made some out of steel bars. After playing around with different lengths I settled on 3mm longer. At 4 or 5mm the rear wheel will hit the rear fender on landings due to the longer bones letting the rear wheel sit closer to the fender. This is a very important mod since how a bike handles is of utmost importance on a MX track. You can buy links from Kouba for a 95 and up KDX200 or 220 for $75 (order the KDX1).
The first thing to do is replace the exhaust pipe unless you are really tight on dough. If so, then just take the power-robbing insulation out of the body of the pipe (not the header. see http://justkdx.dirtrider.net and click "tech tips", then "pipe mod". This mod is for years '89-'94). FMF has pipes for better low end and better top end (35 (torque) or 30 (rev) gnarly series pipes) but many have complained about quality control problems and mounting points not being correctly located. I chose to use the Pro Circuit pipe which enhances the high-mid and top end power. I have been happy with it. Recently I have gotten into expansion chamber design and here is a page explaining how I would custom make a pipe for it: click here
Next things to do is replace the stock reed valves with fiberglass Boyesen Super Stock Fiber Reeds for $30 or Boyesen Power Reeds for $30 or carbon fiber Boyesen Pro Series reeds for $45. Carbon fiber reeds are lighter and open/close faster which allows more air/fuel in the engine at high RPM.
Increasing air intake and preventing water intake
Taking off the stock "snorkel" allows more air intake but also will allow muddy water in (that gets splashed up there) which can cause your throttle to stick open. People do it to allow more air in which aids power output. But the snorkel is there for a reason. Too bad they didn't design it with a more voluminous intake. Once the muddy water cascades into the air box the air filter will pass it in because it only catches "dry" dirt particles with the air filter oil. The muddy water gets up there when it gets tossed "up" there while you are splashing through muddy water. My solution is to toss the restrictive snorkel and cut out a bigger rectangular hole. Then use something (I used cut out pieces from a quart sized mineral water bottle) that you can duct tape in place to build a 1/2" tall "wall" around the big hole to prevent creeping water from coming into the air box. Also you'll need to duct tape the front edge of the cover to the air box below. Also run a piece of duct tape from the left frame tube to the right one (taping to their undersides) in front of the air box which makes a splash guard there (behind where the frame makes a "Y"). Also tape the right upper side of the air box to the frame, effectively making a splash guard for that vulnerable side. These two mating surfaces let splashed water in. Also put a plastic splash guard under the area that's open between the tank and the frame. Also if you ride through deep streams you should silicone seal the air boot to air box juncture. If you have already drilled holes in the air box lid for greater air flow then this fix won't work for you and you'll need to buy another lid (they are cheap enough) and do it right. After all these fixes my air box still would ingest a little muddy water (but much less than before) from going through muddy puddles. So the final refinement was to put a piece of cardboard (sealed from water by putting packaging tape all around it) from the forward air box opening (secured in place with duct tape) to the near-bottom of the air box which basically provides a "chute" for the water to go down without getting onto the filter. Also I made the sides of the cardboard (about 3/8") bend up by 90 degrees (and held in place with tape) so that water won't get sucked over the sides of the cardboard and onto the filter. That way the muddy water just goes all the way down to the bottom of the air box. I finally put my noggin to work on this problem after I lost the last MX race due to the throttle sticking open with dirty water getting in. It was disappointing because I was going to win and it was going to be my only win of the year. I got 2nd place the first moto and then pulled off the track with this problem while in 1st place in the second moto. I was all prepared except for this one weak spot - water intake prevention. Also I've put silicone seal on the top of my carburetor to keep water getting in there where the cable sheath rests against the carb top. There's a rubber boot there but I think water could still get in.
My '89 model has ignition timing set at 21 degrees BTDC at 6000rpm whereas the years after had it set at 19 (with a resultant 1hp gain on top). After hearing that you can change it (if you have a flywheel puller and holder) to enhance performance I experimented by advancing the stator one notch CW and then CCW. CCW gave dull low end performance with just a little more power on top (but nothing significant), and adjusting it CW (advancing the spark) really gave it more umph in the low and mid range without detracting more than a little bit from the top. With it advanced I really like the increased power although it runs a little hotter. So with this mod you need to have a clean cooling system (use radiator flush) and use 50/50 antifreeze/distilled water to prevent coolant loss due to boiling.
Later I started tinkering with changing the timing curve by changing the CDI. (I'm an electronics technician.) I came up with two internal changes that made more power all through the rev range and even extended the rev range. Check it out here.
This mod is to help intake flow from bottom dead center to where the skirt raises enough to allow air/gas flow under it from the reeds. This is done by drilling a few holes in the intake skirt. The stock piston has one hole in the middle with its center 38mm down from the piston top. I enlarged it to 3/8" (9mm) diameter. I also made two holes 3/4" (19mm) to the left and right of the intake centerline and 34mm down from the piston top, that have a 1/4" (7mm) diameter (which I may enlarge to 3/8" (9mm) diameter). And I made one little hole in the centerline (22mm down) with 3/16" (4.5mm) diameter to aid circulation under the piston crown which combats heat buildup there. The 3 main holes help with mid range power (to the best of my knowledge). A wiseco piston comes with one large hole in the center of the intake skirt.
KIPS and compensative engine mods
The power valve system on earlier years had a problem with valves breaking or the gear teeth stripping out. I was replacing mine twice a year so I just glued them (the two side aluminum valves) in the "open" position with epoxy glue. The middle valve still works well since it's made from steel. That change caused some low-to-mid range power loss but I got that back when I epoxied the two exhaust ports (farthest from the main exhaust port) closed. I used the epoxy that comes as two different colored putties. Squish it in as far as where the two little ports passageways meet and then move the piston (with rings in place) up/down to cut off excess putty which would later hit the rings during use.
I have found that 13/48 is a good tradeoff for sprocket sizes. That is because 13/49, or even better 13/50, is best for MX tracks that aren't too fast, but it's hell to get a good start with that combo (for me at least). If the race is long enough so that the start isn't too important then 13/49 or 13/50 (the lowest gearing of the 3 and best if most of the power is at top revs) would be preferred.
I prefer a plug with a fine rare metal center electrode. I gap them just a tad wider (.032", .8mm) and feel it does help the power output (mostly felt at low-mid revs). I use the NGK BR8EIX plug.
This is mostly personal preference, but if you are riding on dry hard ground then I'd recommend the Dunlop 100/90-19 D739 rear tire (which is for a 19" rear wheel). Any good hard terrain front tire will do. I run with 15 lbs pressure up front and 12 lbs in the rear. Two other good tires are: Maxxis IT M7305, Dunlop MX71
Dressing for the crash
Hey, its part of MX, crashing. Your feet need a good pair of boots but it seems that the boots with the best protection are actually somewhat dangerous because they limit your ability to control the bike because they are too stiff. I have Oxtar Raptor boots and I had to cut the ankle plastic on the left ankle to be able to shift. So be sure to look at the outer ankle area of boots to see if it's leather or plastic (too stiff). For knee protection I recommend the AXO knee protectors ($55 from MXSouth) which also protects the tender area just above the knee. For elbow/forearm protection I recommend the ones made by AXO or Thor.
Click here if you want to see my page about the international SR version of the KDX200.