Murry push mower rev high,rev low,rev high,rev low

Possibly two things. Either the carb is dirty causing it to run lean, or the governor springs if it has the two spring system are out of sync.
 
It requires a tachometer to due properly. Take the tension off the small spring and adjust the big spring to 200 rpm below the correct speed for your engine and then adjust the small spring to the final rpm setting.
 
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