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I have a Bolens st120 lawn tractor. It has a 12hp briggs I/C motor. The person I got it from ran something over and it wouldn't restart. The motor is backfiring out of the carb. This is my first small engine project and trying to learn as much as possible. Any help would be greatly appreciated... sorry about the dog bark in the video.



Someone told me to check the flywheel key so I striped it down and took a few pics since I dont know what it should to look like.


 

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I removed the flywheel and the key looks good and was seated correctly between the crank and the flywheel.
 

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The problem most likely is in the valves. I would pull the head and check the valve seat, make sure it's not loose in the block. If that looks good, then check the valve lash. L-Head engines can loose clearance over time on the valves.
 

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What is the proper lash ? I believe I read somewhere that it should be .005. Also is it possible to check all this on the mower or should it be removed ? Thanks for all the help
 

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What is the proper lash ? I believe I read somewhere that it should be .005. Also is it possible to check all this on the mower or should it be removed ? Thanks for all the help
You can use the Briggs and Stratton check chart to find settings for most of their common engines. Since you did not post the model, type and code numbers off your engine, this is your best bet.
 
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