No need for "flooring" these engines.

They should transition from low to high with no hesitation; however, the throttle stick shouldn't be moved suddenly.

I would check that washer that Opjose mentioned and the balance of the prop, besides all the bolts.

Opening the needle valve always helps at starts and for extra cooling and lubrication.

Study these links:

http://www.gettingairborne.com/engines.html

http://masportaviator.com/category/advanced/engines/