Building Your Own Secret Weapon,

1989 - 94 KDX200 Pipe Modifications

By Canadian Dave

If you have a stock expansion chamber on your 1989 to 1994 KDX200 chances are it’s a little worse for wear by now. (It doesn’t normally get called and expansion chamber and its long to type so lets just call it a pipe like every one else.) Well you’re in luck. You can remove the dents and modify it into a very serious performance pipe all at the same time. There’s a powerful beast lurking in that factory pipe and these directions will help release it.

These instructions will take you through a step by step procedure to remove the inner layer. Now this isn’t a job for the faint of heart there is cutting and welding involved here. However the end result is well worth the effort. Many people prefer a modified factory pipe to even the FMF pipe.

When you cut open the factory pipe you’ll soon discover why its called a double walled pipe but you might be surprised what the second and third layer consists of. The inside of the pipe’s main body looks very much like the inside of a silencer. You’ll find a layer of silencer packing sandwiched between a layer of perforated sheet steel. The inner layer of steel looks exactly the same as the inner tubing of a free flowing silencer except that it is flat rather than rolled.

All this extra material does wonders to silence the traditional ring-ding-ding sound emitted by two stroke engines. Unfortunately is also absorbs/retards a great deal of the ultrasonic pulse that is so important to good two stroke performance. For more information on ultrasonic waves and how they effect 2-stroke engine performance check out my article KIPS Valve Explained.

 GETTING STARTED

The shopping list:

  • stock pipe
  • hack saw with a fine tooth blade
  • scribe
  • Vice Grips
  • large screwdriver
  • hammer
  • access to a tig or mig welder
  • high temperature exhaust manifold paint

GETTING TO WORK

Remove the pipe and set it out in front of you. You’ll notice about 1/3 of the pipe’s length after the U shaped header section there’s a ring of spot welds running all the way around the pipe. If you look about half way through the body of the pipe you’ll see the same thing is repeated there as well. This is where the inner liner is welded to the body and where you’re going to cut the pipe. There are also a number of spot welds scattered between these two rings. These randomly spot-weld the inner liner to the body of the pipe. You’ll need to pry these apart using Vise Grips, a long screwdriver and a hammer as required.

Step One – Have a look at the two rings of spot welds. Using a scribe draw two Xs through the rings. One in the middle of the two factory halves and the other on the opposite side. Repeat this for both rings of welding. You’ll use these markings and the factory welded seam to help you realign the pipe when it comes time to weld it back together.

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Step Two – Break out the hacksaw and start cutting. Cut through the center of the two rings of spot welds. This will make it easier to remove the inner lining because the majority of the welds will be cut in half and you can use the spot welds to keep your cut on track.

Step Three – Remove the inner liner. I found the easiest method was to lock a set of Vice Grips on the liner and start rotating it, twisting it around its self. You’re likely to end up with a few ripped lengths of liner far enough into the pipe body that you can’t lock on to them with the Vice Grips. Here you can use a long flat screwdriver to pry them loose. If worse comes to worse give the screwdriver a few smacks with the hammer. If your pipe is dented here’s your chance to straighten it out. It is handy to have a short length of thick walled pipe to use as a dolly. Place the pipe inside the pipe’s body and work out the dent by striking the outside of the pipe with your hammer while holding the pipe tightly to the inside wall. If you or a friend has some body working experience and tools there are a number of standard dollies that will really help here and the methods are the same. If the thinner, curved header section of the pipe is dented you can normally get in there with a long solid rod and bang or pry them out.

Step Four – Weld that baby back together. You’re going to need either a MIG or TIG welder. If you have a brazing torch that might work too, though I have never tried it so can’t say for sure. Realign the pipe using the Xs your scribed plus the factory-welded seams. Once aligned tack weld the pipe together in a few spots and test fit it on the bike. If every thing looks good stitch weld the remainder of the pipe back together. If it is out of alignment grind the tack welds away and try again until it does fit properly. The pipe is rubber mounted to the frame so you’ll have to be pretty far off before it won’t fit.

Once its welded back together you can dress the welds with an orbital sander and some very course sand paper if you want. Try 40 grit. Be careful though not to grind through the pipe, its thin material. Once you have it looking the way you want break out the high temperature manifold paint. I like the UHT brand Cast Iron Look paint. It’s a nice change from flat black. Just a hint here, high temperature paints are heat cured. If you jump on the bike and ride it before the paint is cured branches etc. on the first ride will scratch the paint off the pipe. After you install the pipe run the bike a few times and let the pipe heat up fully. This will allow the paint to harden before taking it to the trails.

You’ll need to rejet your carburetor after you install the modified pipe. For a starting point check out the Carb Tuning Guide.

Enjoy your new factory look sleeper pipe. Your friends wont even know its coming.

If you have any comments or suggestions smack the e-mail icon and send them my way or visit the JustKDX Forum. email@.gif (25222 bytes)

Happy KDX’n,

David