OHV175 tecumseh engine problem?

I have a stanley lawnmower made by murray with the OHV175 engine in it and sense it was new it has run pretty rich, backfires when u shut it off unless u let it idle for a minute before u shut it off, and also it lopes pretty bad when its cold, its not lope but can't think of the term for it. speed goes up and down. after it warms it don't do it anymore. any ideas on what to do to fix this? It only has 1 adjustment screw and its the idle set. If it would have just started this it might not be a problem but its been doing it sense new. thanks for ur time.