First lets determine what the term “locked up” actually means. When an engine is locked up, it means that the crankshaft will not rotate and is stuck or locked up in that position.
Is the engine low on oil?
There are different things that can cause a lawn mower engine to get locked up and the first one is a lack of oil. When a lawn mower comes into our shop one of the first things we do is to check the oil and if there is none on the dipstick or it we barely see any, it usually is not a good sign.
Running any engine low on oil causes the bearing surfaces to not get any lubrication and they become very hot and wear quickly. If they become hot enough, they will actually weld the connecting rod to the crankshaft or the piston pin. In that case, the engine is most likely junk and it is cheaper to replace it or the lawn mower.
If there was a small amount of oil on the dipstick, the engine may have only partially seized up and is still usable. It will just need to be broken loose by removing the spark plug wire and gently turning the blade on a push or self-propelled mower. If it breaks loose without a great deal of effort, then you will want to add oil and run the mower and then change the oil again. Running the mower for a while to ensure that it is not making any knocking sounds or is down on power is important as you don’t want to throw a rod out of the block.
Are the push rods connected to the rocker arms and adjusted correctly?
An engine with loose push rods can cause the engine to lock up. One push rod will fall off of the rocker arm and then the cylinder will have high compression and no way to relieve it.
Is there raw gas in the cylinder?
A cylinder can also fill with gasoline that is leaking from the carburetor and cause the engine to hydo lock. You turn the key and try to start your riding lawn mower and it turns 180 degrees and then stops is a sign of a hydro locked mower engine. First you need to repair the carburetor that is causing this issue and then remove the raw gas out of the engine cylinder.
Is there something that is stopping the blade?
Other things that can make the engine lock up are a large amount of grass under the deck or a rock or stick caught in the blade. Check under the deck and be sure to pull the spark plug wire off before you do.
Is your pull starter or electric starter broken?
Your pull starter or electric starter can break while it is engaged and cause the engine to lock up. On a riding lawn mower you can remove the blower cover and see if the starter is stuck in the engaged position. You can do the same for a push or self-propelled lawn mower.
Is something inside the engine broken?
This is something that is an unknown because you cannot see if the camshaft or something else has broken and is binding inside the engine. This will require disassembly to determine what is damaged and needs to be repaired.
By carefully looking for these clues, you can determine why my lawn mower locked up.