Small Engines (Lawn Mowers, etc.): starter turns but the engine will not, bolens tractor, time strip

Question
No I checked that, the starter runs but will not engage the engine to turn over.
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-----Question-----
I have a Bolens tractor with 18hp Kohler engine. I replaced the silinoid, the starter still makes noise but does not turn engine over. I used it last week fine. Any thoughts?
Thanks
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battery dead?
 even if you used it last week if charging system in engine is not keeping battery charged?  it will give problems

Answer
okay  then  i,d say the starter gear is worn  out.  they are normally plastic, and over time strip the gears which ingage to fly wheel.  off. and you  need to replace the gear. you can see this if you remove the small cover over top of starter mounted to flywheel cover.