pull the plug and see if it actually has spark, did you make sure all your seal and gaskets were good..... Even when you just tear your motor down it's always a good idea to replace the gaskets, check if the carb is gummed up, also take the whole intake assembly off and cover up the carburator creating a seal with your hand and then crank it..... I'd should have a vacuum feeling on your hand and should start like this if it has spark and gas getting to it, and if the carb is nice and clean..... Just some basic easy suggestions.