The lawnmoer starts with starter fluid but won't stay on

How fresh is the gas. Old gas can be a problem. When was the last time it was run on a regular basis. If it a spring start so to speak how full was the lawn mowers gas tank while sitting? Check and clean plug. If its to black and build up with deposits too much get new plug. A dirty plug won't have a hot enough spark to ignite gas but enough to ignite starting fluid.

BASIC here is fresh gas and clean good plug. Then it to cracked fuel lines and filter to carburetor

Signed 21 Boat

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1. drain & flush fuel Tank. 2. Install new fuel filter. 3. Remove fuel bowl on carburetor and carefully remove the needle valve and spring & clean with a carb cleaner. Remove spark plug & check for proper gap & clean it and install.

I would just add that a clogged air filter can cause or add to the problem also.