Briggs & Stratton wont idle stable, rough low idle and it sounds like it is suffocating. Low Idle Problems With a LawnMower where it is surging and idling low then coming back up can be caused by numerious thing. In this video we troubleshoot and fix a Craftsman 190cc* Briggs & Stratton Platinum Engine, 22" EZ Lawn Mower. The repair is much easier then you might imagine. In todays case the mower surging up and down was caused by the engine running lean and we fixed it by removing the air cleaner to gain access to the carburetor, where we removed the carb bowl and inspected its contents. I found the Fuel to be in great shape so I decided to to do a quick clean of the jet. To clean the Jet I looked down the intake of the carburetor and identified the top side of the brass jet. Then I used carburetor cleaner to back wash the jet and clean it out. Then reassembled the lawnmower and it ran perfectly.

Here are some other recourses for trouble shooting you lawn mower.

Lawn Mower: Engine Rpm's Surge Up And Down
"If your lawnmower’s engine is surging it could be a problem with the carburetor. The carburetor mixes air and fuel to the perfect ratio for combustion. There are passages and nozzles (called jets) that route the fuel through the carburetor. The jets meter the precise amount of fuel that is needed for combustion. The jets have tiny openings that can easily be clogged by dirt or from corrosion caused by old fuel sitting in the carburetor. In the center of the carburetor, near the bottom of the carburetor bowl is the main jet. The main jet supplies fuel to the engine when it is at full throttle. When the main jet becomes blocked, the engine will stall at full throttle. Once stalled, the governor will close the throttle. At this point the carburetor’s idle circuit will begin to supply fuel and the engine will fire once again and the throttle will return to full. This cycle repeats over and over, causing the engine to die and surge. Fixing a surging engine is usually a matter of cleaning the carburetor..."
https://www.ereplacementparts.com/rep...

Does Your Lawn Mower Sound Like It Is Surging? Here Is Why & How to Fix It
"This the most common cause of surging issues. Grime and dirt can gum up the internal components of your carburetor, making it fail to receive the correct flow of fuel. Solution: You can take your mower to a small engine repair shop to have the carburetor removed and cleaned. However, if you’re comfortable with your repair skills, then here are the steps you should take:
1.Disassemble the carburetor. For you to reach the carburetor, you’re likely to have to disconnect the air filter, fuel tank, governor control link, breather pipe, and manifold seal and keeper ring.
2. Clean the carburetor with a carburetor cleaner that’s appropriate for your mower engine, paying special attention to the needle valves, orifices, and ports. You’ll need to replace the carburetor if its body is in poor condition or damaged.
3. Once you’ve given the carburetor a good cleaning, reassemble the mower components in reverse order..."
https://www.brainyadvisor.com/lawnmo...

How to Troubleshoot Surging Lawn Mower Engines
"A Fouled Carburetor Carburetors can also get fouled by dirt and grime that gums up its internal components. You can take your mower to a small engine specialist or remove just the carburetor and take it in for cleaning. If you are comfortable in your machine repair skills, you can disassemble and clean the carburetor yourself. Check the float for pinholes and free movement, and clean the orifices, ports and needle valves with carburetor cleaner approved for your engine. Use a carburetor kit if you find any damaged components or torn gaskets. Replace a carburetor that is in bad condition or has a damaged body..."
https://www.hunker.com/12363720/howt...

Low Idle Problems With a Lawn Mower
"A lawnmower engine will idle around 1,200 to 2,000 rpm. This idling system will allow just enough fuel into the carburetor for the combustion process to continue. When the mower has trouble idling, or idles too low, the mower will shut off. Often this is caused by a decrease in air or fuel to the carburetor...Carburetors on lawnmowers generally need cleaning and servicing only once every few years. However, if the operator left gas in the tank during a storage period, the dried gas may have damaged the fuel system and dirtied the carburetor. The sticky substance left by the gas will cause the idling system to be out of alignment. A dirty carburetor should be removed, disassembled and cleaned. A service professional should perform these duties..."
https://www.gardenguides.com/12574616...