Is this a new engine and are you breaking it in? If so do you have it set to factory settings? Is so you might need to lean it out a little or idle it up. Yes I seal all my engines not matter what brand out of the box. I have 3 STS engines and do not have a problem with any of them. All so here is a web site with a help forum on just STS engines.
http://www.stsmicroengines.com/