Carburettyor cleaner I live and swear by CRC clean-r-carb it is by far the best I have used, shouldnt need any sealer as such just new gaskets....
Before going tothat extreme though, it would be worth to make sure the choke is closing, thats the litle tab on the back of the carby under the air filter, it should be able to pull right out and blockthe throat of the carb. If thats not working you will have a bad time trying to start it.
