DIY 2-stroke Expansion Chambers
For two-stroke riders looking for a cheap, power-producing alternative to aftermarket expansion chambers, try the following: find the point on your existing pipe(s) where the diameter becomes largest (usually a foot or so from the downstream end of the headpipe) and cut it (them) there. Then braze the end (converging) cone and stinger from a junk dirtbike expansion chamber to it. You're usually dealing with a large enough and cool enough pipe that braze will be plenty strong, if you do it right. Try to find dirt bike pipes off a model with a similar displacement to each cylinder of your bike. I had some success doing this to an old GT380 triple and a friend's GT185 twin years ago. I used two RM125 pipes and one that was probably off a YZ125 for the 380. I used all of the YZ pipe except the headpipe, in fact, because the middle cylinder of a 380 splits into two exhausts...all I could use of it was the headpipe. The YZ pipe tucked right around the rear wheel with a little cutting and brazing, and that thing would do easy second-gear wheelies afterwards, even with no rejetting. The 185 gained more top-end but lost a lot of low-end, and ended up holing a piston. Multicylinder two-strokes sound incredible with unmuffled expansion chambers. This approach saves a lot of involved mathematics and should work on most restrictive old two-stroke exhausts, at least with some rejetting, but be advised that you'll trash your original pipes doing it. Might want to try it on a spare set of pipes first, and if you run it lean and hole a piston or two, blame yourself for not jetting it right and not doing enough math--like I did. Good luck!
Ben - USA