Suggestions needed 50cc bike problems

Right I've got another bike to work in today, it won't start, I've had the carb off fully cleaned out, there's a nice spark coming from the plug, but it just won't start I've tried putting a tiny bit of petrol into the head through spark plug hole still no luck, the guy said to me it was running before then he cut it out and came back like 10mins later trying to start it again but I wouldn't go? Any suggestions?