100psi sounds like it needs some attention. From what I have read it should be at least 110psi. Be sure to carefully test the compression. The first time I tested mine, I had attached the extension bit of the guage - which gave me a lower reading as apposed to the direct attachment (see image if you don't get what i mean). Also make sure it's screwed in pretty tight, and kick it a few times.

I've got the exact same problem on the same cylinder (lower) as you do.

I have just been given another regulator/rectifier to swap out and test (Thanks Bryan ) to see if that is the problem. I'll let you know how i go with the reg/rec... when i get around to it

But I suspect it's a blocked slow jet as someone has suggested. A few times when I have come to a stop, it almost stalls and I've had to give it a bit of throttle to bring the revs back up over 2000rpm. I've yet to check the carbs as I don't have a clue about them

Hope you get it sorted.
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Jerry Lai
NSR250 [MC21SP]