Sv150 engine hunting

I have an sv150 lawnmower engine giving me a real headache. It starts fine hot or cold but then starts to hunt. I ultrasonically cleaned the carb twice. And no joy. In the end I opted for a new carb. Fitted this today a still hunting. Checked flywheel key all sound. I did have to replace the blade boss because of the lugs were missing on the old one though the blade was undamaged. I've checked for damage on the exhaust and all good. Do I need to check the head and valves. Is the issue hiding in that area does anybody know please.