A bad, stuck or dirty float needle inside the carburetor.
You probably have a dirty carburetor causing that surge.
you tube how to clean a mower carburator.
does it have spark to the plug if not the part you might check would be the coil or the electronic start module probably the latter if it is a newer mower or of ots older then the coil magneto usually dont go bad could be a bad gap between the coil and the magneto if its an older mower could be a bad set of points if its an older mower point gap could be bad spark plug could be bad as I don't know exactly what the lawn mower is doing hard to tell what is wrong
Ask Someone at Lowes
If you are talking about the bulb button it is a primer to get gas to the carburetor if the mower has not run for awhile. Usually not needed to restart a warm mower.
If your lawn mower is pulsating, it may mean that it is not getting enough fuel and the carburetor may need cleaning.
They are for the fuel solenoid shutoff
Depending on how this mower got out of tune is how you are going to know how to tune it back. It may be as simple as adjusting the mixture screw on your carburetor, or as difficult as filing the points (older engine) or replacing the diaphragms on the carburetor.
Any good lawn mower repair shop should be able to order just about any brand of mower part for you. Good luck.
Carburetor float is stuck open.
It is not fuel injected so yes it has a carburettor