1- How many hours does the engine have? Less than 1 hour is unacceptabe for this engine. It should be run-in at a rich setting to bed the ring and liner.

2- Remove the carb and check o-ring and needles for clogging, reinstall the carb with the o-ring well seated.

Always put a new plug sfter the first run-in tanks as a lot of debris from the running-in process will contaminate the glow plug filament.

My bet is #1, mine runs sweet and has always started first back-flip on the prop.

http://www.youtube.com/watch?v=FL-Z1Iz7S5g
http://www.youtube.com/watch?v=RbGtoVXvHZ8