A poorly broken in engine,
an improperly adjusted carburetor
and/or an improperly installed fuel tank will make your flight unreliable.
Always follow directions and break in your new 2-stroke or 4-stroke engine before trying to fly.
This requires several tanks of fuel and a controlled running of the engine to condition it for proper operation. What you want to do is obtain a reliable idle and a smooth transition from idle to full power. When it comes to power output, always run the engine slightly rich, not lean.
Every preflight check should be making sure the clunk pick up in the tank rattles and isn't stuck.
It was stated you don't have to check your valve clearance after I suggested you do. Not all engines leaving the factory are set correctly. I've had two OS engines (new in box) so far that were out of adjustment in the past year.
I got a lot better idle after correcting it to the specs in the manual as well as throttle response.
http://www.osengines.com/faq/product-faq.html