The neighbors Cub zero turn had a leaky valve cover the end of last year so I told him I would let him know when I had a chance to look at it.
Not trying to start a debate on mouse poison and the risks to pets but if you want to use be aware of this.
First thing I found was mouse poison stuffed in one side of the engine. He used 2 types. One small and thin and one the size of gumballs. The gumballs are like rocks and will not pass through the cooling fins so they would get part way down and stop and then block grass and pine needles. The thinner ones would make it through if the gumballs had not jammed up the works. I could see the issue right away without removing the cover since needles and grass were spilling out of the spark plugs hole on the left side. I told him to check that first before he ran it in the future.
How I found it.
This was not all from the motor but the amount of mouse poison I found as I blew things out was amazing.
The dark spot was the discolored valve color and screw from getting very hot from the lack of airflow.
Set the valves, the coil module gaps, put in plugs and changed the oil & filter, fuel filter and sent it on its way.