I'm having the same problem i cleaned my gy6 engine and now it wont start i checked timing, spark, fuel, have back pressure, starter is new and so is my solenoid.
it cranks but wont start it doesn't burn oil in fact i has new oil what should i do??? it wont even start off carb cleaner it was running before i cleaned it but really ruff.

 

What do you mean by having back preasure, and not burning oil? Carb cleaners are not a preferred fluid for engine start, use start spray/gas.

Mine did also turn over by the starter, but not fast enough to get start, do you have compression? You cleaned the carburetor or the engine?