2 1/2 Briggs......doesn't run smoothly....surges at all speeds.

Check valves. Vacuum from the engine draws the fuel in...sounds like vacuum is surging. Also it may have a "fuel pump" that may be bad

if its a briggs and stratton engine with the plastic carb fixed to top of the fuel tank then you will need to replace the diaphragm and blow out the fuel pick up pipe

This sounds like a problem with the suction. If the air filter is clean then it will need new carb gaskets. There may also be an issue with the pressure being created by the engine.

it does sound like a carb issue it may need to be rebuilt also check your fuel line and if you have a fuel filter check the filter clean the gov. assembly with carb cleaner

clean your carb and change needle and seat also

I am pretty sure this is a 4.5 Hp engine with the carb on top of the tank. Plastic carb 4 or 5 mounting screws mating carb to tank. Remove the air cleaner... and tighten the carb to fuel tank mating screws. It is drawing air between the tank and the carb. This should stop the surging. If is has been a long period that you used the mower with the surging problem the diaphragm between the tank and the carb may be either out of position or damaged. The diaphragm is not expensive. Usually the first time this symptom presents itself you can resolve it by tightening the carb to tank scresw. IF you have the primer bulb carb..mash the rubber of the bulb against the carb body. If the surging stops... the primber bulb has a pin hole in it.

You passed the mower model number and not the engine model number. When we use the 11A number you passed all we get is the base and handle parts. If you need more help I need the model number off the engine just above the spark plug wire on the air shroud.

Thanks for choosing Fixya,
Kelly

needs carb rebuilt on it

Check the governer, its a lever that pulls the throttle plate from the engine side. Try manually overriding it (twist it open as the engine's running) and see if that does it.

check your air filter. If it's clogged, the engine will spool up to keep from dying, run for about 15 seconds, then it will happen again. If that's it, just clean the air filter. Solved my problem

There is still either water or debree contaminating the fuel system. Surging is usually debree, empty and flush the tank with gas or Iso alcohol, disassemble carb soak in solvent for 30 minutes blow out the carb passages, reassemble and if the problem persists run engine at top rpm with air cleaner off and block off carb intake with hand, this will stall the motor restart and repeat 4 - 5 times this wiil draw out any debree or other blockage in the carb.