请帮我啃诊断一下snowblow的问题

I have a 3-year old Toro snow blower. I didn't drain the gas after last use, and it won't start now. I thought the carburetor must be clogged up. While I tried to drain the gas by loosening a bolt under the carburetor as instructed by the manual, the gas won't even drain. only a small amount of dirty gas flew out. Half tank gas won't flow out. is this another evidence the carburetor is gummped up? please help me to diagnose it. I thought the spark plug should be fine given it is only 3-year old.