I told you, try a new piston ring.
There is always a chance it won't help but...It's the only thing that fixes mine.
Surging a full throttle with a 91FZ is a CLASSIC problem.
This has been the only fix I've found with that engine in 10 plus years of operation.[&o]
And yes, it has nothing to do with compression, or at least normal compression type tests... so don't bother thinking about it.
It's about the hot fuel igniting, blowing the piston down and some of the burning hot gasses creeping passed the ring.
Order a new ring and try it, if no-go send it to Probuild for a full service.
Least you'll have a spare ring.
I normally carry a couple, plus complete gasket sets, plus O rings for all my YS engines.
If you've been operating these engine for a long time... you'll know why.