A 2-stroke depends on a certain amount of back pressure from the exhaust system. My Dad would build a tuned exhaust pipe specifically for that particular bike (a pretty time consuming process that I'll explain later, if asked).
Bill, consider yourself asked. 
You talked me into it.
Anyway, it starts like this. Dad took and made straight pipes (the same length as the original exhaust pipe/mufflers were) for that Jawa, welded a flat washer on each one and took it down the street to check it out. When he came back, he cut a 1" piece off of each one, welded a washer back on and took it down the street again. Each time he came back, he would do this and then, he came back and cut 2 of those pieces he had previously cut off, cut them in half and welded them on the end with washers on the ends. Sounds kinda crude I guess, but it worked like a charm. That 250 (which had internal modifications, that I have record of) would really run! When I returned home from the Air Force, I had plans for that one only to find out that (for reasons that I don't know) my Dad had let my Uncle Bobby take and use the Jawa. There was a fire where my uncle was and the Jawa burnt somewhere out in California.
I could not believe it! I had some history with that bike and that's one reason I want to try to recreate it with one I picked up on. Like I said, I've got the Jawa Factory booklet that details the mods and includes pictures along with drawings. That bike didnot run like a typical 2 stroke of that time period. No hesitation, just open the throttle and GOOOOO!!!!
BTW, he also made upswept pipes (Scrambler Style) He decided how he wanted the pipe to bend, he would mark ever so often along the area that was to be bent, he then made a slender cut (like a "pie slice", or wedge cut) on each mark, then he would bent the pipe to shape, weld up all the cuts, check the pipe fit, paint it with Stovepipe Paint and call it done. BTW, all the welding was done with an Oxy/Acetelene (carbide type) welder.
Later on, Bill
Forgot to mention that some of those trips down the road (on a bike with straight pipes) would be cut short cause Dad had to get back to the shop before the cops caught him!
Somehow, he always managed to get back, get off of the bike and look
SO innocent with the cops KNOWING that it was him that just made that run! On
that note, sometimes a couple of guys would get into a heated discussion on whose bike was the fastest and the next thing you know, they were lined up in front of The Shop (at the cross walk), my Grandfather would stand in the front door of The Shop, drop a shop rag and the race was on!
There were some pretty interesting races
that I remember! On occasion, a cop car would happen along as the race started and
that was always interesting too!
Ah, Those Were The Days!
Later on, Bill
