hi,
another question that i hope you clever people can help me with.my nsr runs almost cold on the front cylinder when idling but heats up ok when you give it some revs,it has a cracked muffler on that side but i didnt think that would cause this problem.has anyone had this issue or know of any common problems like this?i am waiting on a new mufflers from tyga so i can start using my new bike.

blocked slow jet.

I have the same problem but it's the rear cylinder that's running cold.
Also if you feel the exhaust from both pipes by placing your hand close to the exhaust cans, the pulse is much stronger from the front cylinder that the rear at idle.
The left exhaust(rear cylinder) has cold exhaust and a much lower pulse, it also sound like it stutters every 9-10 seconds and the returns to normal.
Can you check this on your bike an post results.
The mechanic said he balanced and cleaned the carbs and checked the coils and plugs. All were OK.
He noticed it too and tried to even them out but when he got close the bike ran very badly so he set the carbs back to standard as per manual.
I posted on this some time back but I can't find a satisfactory answer to it.
I don't expect it's normal as I would expect both cylinders to run at even temperature.
There's something not right some where but I can't find where or what could be causing the problem.

hi
i have not been able to test mine much since unblocking the slow as i found the pistons were scored then when i took it apart i found cracks on the exhaust ports so it is still in bits,but the brief run i did on it after i unblocked the jets seemed to fix my problem.sorry i am not much help,i have only had the the bike a bit over a week and only rode it a couple km to get it home and found all these problems .

I would do a pressure check to make shore the crank seals are ok . When my crank seal went one cylynder was not firing (cold exaust) at idle but would at higher revs.

I didn't think of leaking crank seals. Jeees, I hope it's not that as I don't fancy striping the whole engine and getting a new crank. How do you do a pressure test on each section of the crank cases? Is this the same as doing a pressure test on the cylinder through the spark plug entrance?
Both cylinders are firing at idle although the cold one seems to stutter a bit and doesn't have the same powerful pulse as the front cylinder.
Fovu, you're a lucky man it's only a blocked jet. I know the jets are not a problem on my bike as the carbs were stripped and all jets were all cleaned. All else seems to be good aswell so it looks like it could be a crank seal leak.
How did you do the pressure check?

check out this thread http://www.nsr250.net/forums/viewtopic.php?t=2220&highlight=pressure+test . Another way to check is to take the carbs of fill the top cylinder crankcase with petrol and leave for a while(overnight) and see if it leaks into the other side not as good as pressure test but should will tell you if seals leak a bit

baron 580
i am not that lucky,looking for new barrels and need new piston and rings and have yet to enjoy the bloody thing.

Ya, putting it that way, maybe not so lucky. You could get the existing cylinders Nicosil re-plated if they're not badly damaged. Pistons and rings are available.
I know new cylinders are very expensive. They can be got from Andy in Japan but given the current DEVESTATION over there right now it might be some time before anything comes out of Japan. My heart goes out to the Japanese people. It's just awful to see that level of destruction and suffering.

What ended up resolving your issue. I'm having the same thing happening.
Thanks jesse.