Lawnmowers use internal combustion engines, which, just like car engines, need to be maintained regularly or they will start to run poorly. One way to tell if your mower is running off key is surging. This is when the power of the mower goes up and down with no manual control. There are ways to troubleshoot this problem to find out exactly what the problem is.
Check the cable to the throttle control. Pull it back to see if the throttle is loose. If the throttle is loose, the engine is running too lean, which means it's not getting enough fuel. This will cause the engine to surge. Replace the cable.
Disconnect the spark plug. Lay the mower on its side. Check for a large amount of grass build-up. Remove any grass you find. This is holding the blade back.
Remove the air filter. Examine it for dirt. Replace the filter if it's dirty. If it isn't, start the engine without the filter on the mower. If it is still stuttering cut the engine off.
Remove and replace the spark plug. A bad spark plug will not fire correctly, causing the engine to surge.
Spray carburettor cleaner into the carburettor, which is located where you removed the air filter. Place the spark plug cover back on the spark plug and start the mower. While the engine is running, spray more cleaner into the carburettor. If the engine is still surging, the problem is with the pistons or the cylinder, and the engine will require professional maintenance.