RE: YS 120 ???
Sounds like either a stuck valve or ring. I'd put my money on the stuck valve; it is common on engines that sit..
Pop the rocker cover off and see if one of them is stuck down. A little penetrating oil and pushing on them a few times will often free them up.
If that's not it, put a few drops of oil in the cylinder through the glow plug hole and turn it over a few times.