Did you replace the silicon regulator when you rebuilt the motor? When you put it together, did you use any petroleum based lubes? If you did, they might have harmed the reg. I have seen those regulators go bad, even though they are just a flat piece od silicone, they do go bad. I assume they get stiff. This may sound dumb, but are you letting the motor warm up? Every YS I have ever run will detonate when cold, some to the point of kicking the prop off. Let it idle for about a minute, esp on the first start of the day. get plenty of heat in the motor before trying to advance the throttle. As far as setting the needle, You need to set it rich enough so you can get the motor up to wide open throttle, then needle it to it best RPM. YS are not exactly quick to react to needle valve changes. Turn it a click or two, then wait 10 seconds. With the long path the air/fuel charge travels, it takes a bit for the needle changes to take effect.