The governor is stuck, causing the engine to over-speed and not return to regular idle speed. You can check the travel of the governor by moving it back and forth with your finger - there is not much travel, but there should be some. Check for debris or accumulated gunk at the lever where it joins the engine and clean it well. Ensure that the engine oil is clean and at normal level. A stuck governor will make the engine over-speed so fast that it can cause the piston rod to break. Good luck.
why does my 4.75 BRIGGS & STRATTON lawnmower engine idles high and low when cutting my grass
Erratic idling, surging, and the "up and down breathing" are usually caused by a dirty or clogged air filter.
Acceleration Cable broken broken, you dump!
Adjust the idle screw.
If its like my escort at the mo then it idles badly when warm and tries to cut out when pulling off.
zyou tps might be bad
On my Chevy truck it idles high when its low on antifreeze but if its not that more than likely its usually a vacuum line loose or cracked. good luck.
If the engine idles and revs up okay in neutral, it is likely the torque converter clutch lock up solenoid.
Need a little more info. What Model and year of the vehicle?
Needs a Tune up
Run a compression test to verify the condition of the engine.
Year, make and model would help.