that's the air cooling passages, the YX150, YX160, Z155Ho, Z160Ho all have them.
have a look behind your spark plug, there is a hole out next to the manifold and another under the head near the exhaust too
normally the casting edges block them off.
Thanks guys, hadn't thought of that. The casting was v rough around the edges and looked like something had punched thru. Great forum, I'd thoroughly searched the net and couldn't find any mention of it.
Checked and yep it connects thru To where the spark plug is. Cheers