The engine won’t work properly if the air filter is dirty/clogged, the spark plug is fouled, the fuel is contaminated/old or the silencer is damaged/clogged.
It takes a lot of power to cut very long grass. The rule of thumb is not to cut more than 1/3 of the length of the grass. If the problem persists, we recommend you contact your local dealer. A list of dealers is available under Service locator above.