Troubleshooting a lawnmower is a relatively simple practice that the average homeowner can accomplish. When you are troubleshooting a problem that is as large as a bad gasket on the engine, the telltale signs should allow you to make a quick determination.
Examine the mower for any signs of oil leaking from the gasket. Depending on the style and brand of the mower, the head gasket will be in various places. If the gasket is leaking or blown, there will be a line of oil around where the two parts are held together.
If the mower is dropping out or not starting and you do notice the oil leak, you have a bad gasket. If the mower shuts off when you turn corners, but there is no oil leak, you may still have a bad gasket since the pressure to keep the combustion going is not strong enough.
Smoke From the Exhaust
Smoke in the exhaust is one of the obvious signs that you have a problem with your combustion system. Because the gasket is the first thing to go and the easiest to replace, this is where you should start. The reason the mower will smoke when the gasket is bad is because oil is filling the chamber where the fuel is supposed to fire.