Small Engines (Lawn Mowers, etc.): lawn mower runs at low power, piston rings, blue spark

Question
Some Background:
I always put preservative in the gas and I always drain the tank by running it out at the end of the Summer.

Started up the lawnmower after it sat over the Winter.  It started right up but runs at about half speed.

Briggs and Statton engine, mulching mower.

Question:
What could cause a lawn mower to lose power?

Answer
Hi John,

I would check the spark plug for a good hot, blue spark.  If that is okay then I would test for a lack of fuel flow by squirting fuel into the carb opening to see if the engine responds.  Compression is another area to check.  Sometimes the rings stick in the piston rings after sitting.

Let me know how you make out.
Michael