125-200cc '12 CR125 Engine Speed Increasing

So this is my first post to this board (which has been unbelievably helpful when I'm seeking information!) and after searching through the forums I haven't seen an answer to previously.

While on a trail ride this past Sunday my sons CR125 began having an odd issue. After a hill climb we stopped for a moment and his engine began revving to the moon with any throttle input. He shut it down and I went over to check things out. Inspected the throttle tube and cable, no binding, pinching, or stretching. Looked around the carb and nothing was out of the ordinary. Started the bike again and it started normally but then began creeping up in RPM on its own once again. We nursed the bike back to the truck since this signaled the end of our riding day.

Could this be a clogged or restricted jet causing the issue? This bike is stock other than an FMF spark arrested slip on, which hasn't caused any issue before. Any advice where I should start checking for problems?

Thanks in advance to anybody willing to post up suggestions as to where to begin looking for the cause of this issue!

-Dan

i'll second both of those look closer oh and check the filter screen on the petcock

Thanks for all the quick responses, guys! Motosportz, we haven't ran it since then but after it started doing that on the ride Sunday is continued. Every time you pulled in the clutch the motor would start revving. I will start it again tomorrow and see if it persists. I'll also check the reed cage bolts and intake boot clamps as well as tap the float bowl if it does it. Flyingbob, it doesn't idle fine. When you let off the throttle it just keeps climbing in RPM. Also, when you start it without touching the throttle it would start increasing RPM. I'll report back tomorrow with my findings. Thanks again, everyone!

It's lean, for some reason. air leak (crank seals, intake manifold/reeds) or a fuel issue (float height, clogged vent line(muddy?), clogged jet, debris on needle or slide(yikes)).

I noticed on my brand new WR125 that the vent lines on the carb were cut square. They should be cut on an angle to prevent mud buildup from blocking flow.