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Problems With a Cub Cadet SR621

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The Cub Cadet SR621 is a push mower and develops common problems over its lifespan. Most troubleshooting techniques can be found in the operator's manual supplied with the SR621 when it is originally purchased. Service manuals also come with the new mower, but if the manual gets lost, it can be purchased through the Cub Cadet customer service.

Does Not Start

A Cub Cadet SR621 that refuses to start can be experiencing a number of problems. The blade control handle must be engaged before the SR621 will start. A faulty spark plug or spark plug wire will not allow the engine to start. A clogged fuel line will prevent fuel from getting into the carburetor or a flooded carburetor will prevent the engine from starting. Always check to see if the fuel tank has gasoline.

Erratic Engine

Only so many things will cause the engine on the SR621 to run erratically. Dirty fuel, water in the fuel and stale fuel will cause the engine miss out or throttle up and down. A carburetor that is not adjusted properly creates the same symptoms. Ensure that you check the air cleaner and spark plug wire if the engine begins to run erratically. A dirty air cleaner or loose spark plug wire also cause engine problems.

Poor Idle

A SR621 that idles poorly can only be caused by a couple of problems. The carburetor requires adjustment or a dirty air cleaner creates this problem, but one other items must be checked if neither one of these items is the cause. A spark plug tip where the gap is too wide will also cause the SR621 to idle poorly.

Uneven Cut

The Cub Cadet must be adjusted properly to cut the grass evenly. Ensure that each wheel on the SR621 is set on the same notch or position. A dull blade will also cause the mower to cut the grass unevenly. A dull blade will also not allow the SR621 to mulch the grass correctly.


Overheating is a common problem with the Cub Cadet SR621. The engine will overheat if it is run with a low oil level. Check the dipstick to ensure that enough oil is in the Cub Cadet. A carburetor that is not adjusted properly will also cause the engine to overheat under normal operating conditions. Check the air cleaner and airflow to the SR621, because if the air cleaner is dirty, it can restrict the airflow into the carburetor, causing the engine to run hot.

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