Per my DH: There is probably some old gas in the carburetor, even if you have new gas in the tank. He says, pull the air cleaner tube of the carb. end and spray some kind of carb. cleaner into the carb. Let it sit for 10 min., reattach the tube and see if this helps. If this doesn't work you may need to take it to the dealer to clean the carburetor. (We have a Yamaha too.)
For winter storage, shut the gas off and run until it dies. Pour some 'Stabil' in the gas tank for the winter.
Let us know how it goes!