Hi I have a 10hp briggs and stratton L motor in a murray ride on mower. It won't start so took the head off and both valves are partly open at the same time hence blowing back through the carby.. I have the valve clearances my question is how do you adjust the valves?. when the motor is turned over by hand there is no clearance at all.

Hi,
Your description sounds as if there has been some form of dirt ingestion wearing the valves/seats to remove clearance. You will have to remove the keepers on the valve stems holding the springs on. Then remove valves, springs and keepers. You must gring the end of the stams to obtain the correct clearance. Then reassemble.
Peace,
Clarke