Winterize the lawn mower

• Remove the blade by removing the nut that attaches it to the mower.
• If there is any rust on the blade, remove it with steel wool or a steel brush.
• Clamp the blade in a vice, or to the edge of a solid work surface.
• Run a file down the cutting edge, following the same angle already on the blade.
• If the blade is badly damaged, you will have to use a bench grinder or have it professionally sharpened.
  • Run the blade perpendicular to the grinding wheel to remove any nicks or gouges (this will give you a straight blunt edge).
  • Hold the blade at the proper angle (the angle already on the blade) and grind the length of the blade until it is sharp.
• Balance the blade (from the center point) on a nail or on a Blade Balancer (~$5) to check that the blade is balanced.
  • If the blade is not balanced, you will have to remove a bit of the material from the heavier side (Do not remove this material from the cutting edge).
• Now that your blade is sharpened and balanced, reattach it to the mower.

Sharpening the blade more frequently will make the task quicker and easier, since you won't have to grind as much to get a good edge.

If you do sharpen the blade using a power grinder heat from the process can weaken the blade, so you'll want to quench the blade to prevent overheating. dip the blade in water frequently during the sharpening process to cool it, and wipe it dry before you continue grinding.

I doubt that your real problem is burnt oil sitting in the engine - the most likely way for oil to be burnt and causing smoke in the exhaust is if the rings are bad and/or the cylinder is scored (possibly because of running too long without enough oil).

You can get a good sense of whether your engine is burning oil by checking the spark plug and looking for deposits or other fouling.

If you're really worried about the oil (i.e. if it's extremely dirty or diluted), change the oil (don't just top it off), then run the engine long enough to warm it up, then change it again.

If the engine doesn't run well with new oil, an engine overhaul is probably your best bet (and usually not that hard to do with a single cylinder small engine if you can find an overhaul kit for it). If the cylinder is scored you might be able to have it bored if you can find an oversize piston and/or rings.