Hey Doug (voodoodb), Long time no speak. Hope all is well with you and yours

Doug (Super08) is pretty much on the mark

Even though I am not supposed to discuss this brand on here, I will tel you where to look on an "Identical" engine

Is it the one you got when you was still in TEDA? I have seen a number of similar problems with them. It is likely one of 3 things.

The front bearing is on its way out
The carb block is leaking
The hub has slipped and the timing is out. (LEast likely as most of them were keyed but I did have a couple of them shear the keys out)

Try the carb block first. They had plastic threads in the insulator block to hold the carbs on in many of them - it is possible the threads have loosened and you need to put in helicoils to tighten up the carb. Also sand the sealing face of the insulator flat.