OK guys this is my first post .I ve been around all the NSR net places for a year but I just recently got me an Internet connection.

Im the proud owner of a MC21 SE that unfortunately is seized for about a year now. The previous owner had severely abused it and I got it immediately in the shop for a full check/tune. Unfortunately money dried up just before we got to open the heads and check their status.

So I got it on the road properly jetted and all and after about a week it started coughing at about 9k, something like half second pauses. After a day or so things got worse and the problem popped up at 7k and the bike died for 1,5 sec at a time. The bike was my daily driver so I had to go and come from work on it until I had the time to get it to the shop. Things got really serious (major erring ,backfire , very difficult to ride). It was obvious that I was running on mainly one cylinder.

So I stripped the thing and found out that the lower ignition box wire had poor contact, I ensured tight contact and sprayed with contact cleaner. It worked perfectly so I tried to ride it immediatly to the shop so it could be checked out by a pro. After about 10km or so, out of the blue, unfortunately the bike seized and was put in my grarage until now.

I recently opened it up and the lower cylinder is scrapped and the ring also. So my question is WHY? I'm about to rebuild with a used lower cylinder and a set of new pistons/rings.What I should look out for?I m told that I should jet a little bit richer for reliability reasons.I also got me a set of JHA pipes. Should I break in the bike with the stock ones and then fit the JHA's?

Thanks in advance for the help. Sorry for the long post!