I have a John Deere LA120 with the same issues. The engine is a Briggs and Stratton with 43 hours of use. I use a Champion RC12YC gapped at .030 inches. The first year the machine started great but not so now. My machine also will not start by cranking.
So, I bought a battery booster at my local auto store to add cranking power. I also bought a new battery with more cranking power but neither helped very much. I called John Deere but they don't have a tech service. They just refer you to your local dealer for service.
I visited one dealer who told me the valves are out of alignment. I thought that was a lot of BS. I visited another dealer who told me the same story. He said valves need to be aligned but within a year you will have the same problem. He also said the engine for the LA series is a piece of junk. Briggs and Stratton hasn't made a good engine in 10 years. The LA series is a cheap machine that John Deere sells to the big box stores.
He would not take my machine in on a trade for the John Deere X series which he said was a much better machine. He did give me this advise to start the machine. Lift the hood and turn the intake screen counter clockwise until you feel resistance. It should start. I tried it and I was able to start the machine.
The last time I used the mower it would not start turning the screen counter-clock wise, so I turn it clock wise and it started. The screen is the big circular screen with all the slots in it that sits on top of the engine. When the engine is running the screen is also turning at engine speed. Hope this helps.
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