Your absolutley right, the less disk there are, the smaller the area that the air can exit, thus more rich condition and more quiet. I just found a manual and read up on it, took freakin forever to find it too.

 

Well I got that stock 99 klx300 exh. system and found a few interesting things. The head pipe is the same dia. as the klx 250 but the mid pipe is 3/8 of an inch larger in dia. and the muffler wieght is at least a pound lighter, I know its not a muzzy or a big gun and so on but for the price of thirty dollars and a coffee (talked him down on price) it should be fun to play with and see what kind of differences it can make to this little 250. I see a rejet in the near future

 

That's bound to be a noticable improvement over stock. I've heard the KLX300 stock silencer is not nearly as restrictive as that on the KLX250S, so it should be a win. You'll also feel a pretty big difference due to rejetting, so it's worth it to do that regardless, IMO. Don't forget the airbox crank case vent drilling / and larger snorkle to go along with the more prolific out gassing of the 300 pipe.

 

I just stumbled across a used 300 full exhaust system and am curious, would I have to rejet for the 300 exhaust or would it be safe to ride as is?