What is Hydraulic lock on a lawn mower?

Answer 1

Hydro-lock happens when liquid instead of gas enters the cylinder. As the piston travels up - the liquid cannot compress like a gas would and the engine basically just stops. If this happens when trying to start the engine - usually it's not a big deal. Just need to remove the liquid and clean out the cylinder and crankcase. If it happens while the engine is running (Not as likely) It will do major damage to the engine. Usually more damage than the machine is worth.

Usually with a lawn mower, Hydro-lock occurs at start-up because fuel has leaked past the carb and into the crankcase and into the cylinders.