I recently purchased a used 2017 TC 125. The bike has 47 hours on it, 10 hours on a new piston and clutch.
It runs great everywhere, but right where it comes into the powerband. I'm not sure how to describe what it's doing, kind of cutting out or bogging. It runs great up to that point and after it. But it you keep the throttle right at that mid position it continues to get worse, add throttle and you're on the pipe and all is good. Replaced spark plug just to rule it out, plug is a light brown after a few laps.
Initially I thought it was the 45 pilot jet that was stock on the 2017. Switched to a 35 to match the 2018 spec, didn't make a difference. It has a 480 main, stock needle and I've been all over the place with the clip position, doesn't seem to cure the problem. With the 45 pilot it would load up in the mid-range, now it seems to run much cleaner but is still cutting out.
Powervalve seems to be functioning as it should and not sticking when revving it on the stand. The powervalve was adjusted to about 4 turns in from full out, changed it to 2 turns. This definitely made the power come on sooner and smoother, but it's still acting up.
Finally I pulled the reeds out. There are some pretty big gaps - could this be the issue? I would have thought reeds would affect the bottom or the top, not the middle but maybe I'm wrong. When I pulled the reed out there was quite a bit of gas in there that came out with them.
Pictures show the reeds sitting on a flashlight. The gaps are twice the thickness of the reed in places.