Could be a dirty carb, could be a leaking crank seal, sticky power valve, plugged exhaust, partially fouled spark plug, plugged air filter. Start with the simple things first. Is the fuel fresh ? Draining the carb for storage is not good enough. There is always fuel left in the bowl and passages that you can't drain out. You should always use a fuel stabilizer to prevent gum and varnish deposits from forming in the carb. These deposits can cause poor engine performance, or even cause the engine to not start at all.