Lawn mower won't start?

I have a 6.5 hp push mower that has always run perfectly but now it suddenly won't start. I've removed and thoroughly cleaned the carb and jets, I've replaced the spark plug and checked for a spark, I've cleaned the gas tank and it still won't start. If I put starting fluid in the spark plug hole, I can get it to start for a few seconds, then it stops. It's getting gas to the carb; if I removed the bowl, there's gas in it. Any suggestions?