If smoke comes out of your mower’s exhaust, this can signify that you have lent your mower on its side to clean underneath it or to check your blades. Now it has some oil in the carburettor. However, if the smoke does not clear up after a minute or two, you have a bigger problem. You may have worn piston rings or a damaged cylinder.
If you need to look under a walk-behind mower, tip it back towards the handles. This is normally a safe way to tip your mower and not get oil into the exhaust or carburettor. It can be useful to get someone to gently hold down to step on the handles while you do this.
A damaged cylinder can be the result of not cleaning your air filter. Dirt may make its way into the cylinder, your rings could just be worn out, or you may have run your engine with too little engine oil. Regardless of the cause, depending on the severity of the damage, it might be time for another engine, and it might mean that you should be looking at a new lawnmower.
If the rest of your mower is in very good condition, you might want to think about a secondhand engine or even a new engine for the mower, or even get us to hone your cylinder and fit new rings. However, any of these can be expensive.