Hi Folks.
I'm new here. Hoping for a bit of advice from you all.
I Have a 2015 Sinnis Max 2 that's randomly developed a carb problem.
The bike is completely standard apart from the stupid SAI thing has been removed and the ports plugged up, and the exhaust is a GS125 pattern part.

I bought it second hand and gave it a thorough service etc. It ran perfectly for a couple of hundred miles, now the revs have started hanging at around 4000.
It starts fine, idles at around 1300, plug is a good colour if not slightly rich.
Throttle cable is new and in good condition, no binding at all.
Carb has been stripped and cleaned 4 times at this point. New o-rings fitted.
Valve clearances are bang-on.
It's a mikuni carb, one of those cheap single cable jobs. Jets are standard, 97.5 main and I think 15 idle but can't remember to be honest.

I've checked for air leaks, all good there. I've tweaked the mixture screw, no joy there. It'currently set to about 1 3/4 turns out which feels about right. Any further out and it starts bogging when I blip the throttle.

Any ideas,guys? It's my son's first bike and I need it to be safe for him.

Cheers Folks,
Thanks very much!