Surging at high power on a YS is almost always an air leak. I can hear it a mile away. Pressurized air/fuel mixture building and bleeding off. It doesn't really go lean as it leaks air and fuel, it just loses air/fuel mass, then builds pressure again, until it has enough pressure to blow past it again. ... Surging.

Usually, it is the valve cover gasket. ...But it can be any seal.

Lapping the valve cover flat with 280 sandpaper or finer on a flat hard surface plate works wonders. You will see how the valve cover gasket area is anything but flat when you do this.

I do this (among other things) before I ever even try to run any YS. First thing that gets done out of the box.

Some say 2 gaskets, I say nope. Fix the distorted valve cover, and you will only need one gasket, ever.

Cheers...