Scooter has lost power but fine on tickover

Hi all,
Looking for some advice if possible. Bought a 2014 Baotian Evolution back in April last year, it ran fine and was a lot of fun to ride through the summer. As the colder months came it was quite temperamental to start in the mornings. We changed the spark plug and air filter which seemed to help.
One morning it started but would not rev up at all, trying to rev it caused it to cut out.
Thought this was a air/fuel problem, cleaned the carb out and eventually replaced the carb and inlet manifold as both were suspect. When this didn't solve it we checked and adjusted the valve clearances then replaced the CDI and coil.
This has resulted in what seems like half a new bike which now starts on the button every time but will not hold revs above 4000, it dies unless I let off the throttle. If it is ticking over I can quickly open the throttle fully and it will get up to about 4000 and drop, with the throttle still held open it will drop to tickover and pick up again - sounds like I am sat there blipping the throttle but I am actually sat with throttle fully open.

Can anyone please suggest what may be causing this? Am I looking at a new head gasket or is it more likely to be worn valves, piston rings or something else?

I am really running out of patience and miss riding it. Thanks for reading.

 

I had a baotian ghost back in the days. Even when new it wouldnt start. Or ran really bad. If it had stood in a humid rainy environment. Does it stand in a garage or outside? Solution was to take it somewhere dry and warm. And work med good the next day when dried up

 

Yes, my understanding is these Chinese scooters are supposed to be sound mechanically but the electrics are suspect which is the only reason I changed the CDI - which did make a difference to it starting.

Thanks for the pointer but I have had it dry under a canopy for the last few months so can rule out dampness.