General recomendation for Yanmar mounts on a GM is a 5 year replacement cycle.
I'm reading that your engine is new and only has 300 hrs on the clock. I'd venture a guees that the mounts a) are in the wrong place, b) the idle is set to low.
The mounts are moulded with a numeric value on the side. The larger number on the front mounts and lower on the rear. Run up the RPM
to around 1000, she should smoth out. It's common for the tach to be off a few hundred RPM
. If you use a strobe then you can calibrate the tach with the screw on the rear side. There is an area of speed typicly 1700-2100 that it realy shakes. This is a dynamic of the 2 cylinder. Don't run there. The little GM's love to be reved 2800-3200 cruise
with a one hour rating at 3600.