I have a riding mower with a Briggs 20HP Intek engine. A couple of years ago it started sputtering when put under a moderate load. The problem is worse when going up a hill or in a tight turn. Once the sputtering starts it usually won't stop without depressing the clutch to stop the mower or briefly placing the throttle in the choke position. The sputtering is everpresent but does have periods where it is better (almost gone) though sometimes it is almost unbearable. I have tried cleaning the air filter changing the fuel filter and adjusting the carb. Nothing seemed to make a difference.

Look at the shroud.

you have a sheared timing key it is under thefly wheel replace says the red neck

clean the carb dirt idle cicuit

have you checked all the pipes for blockage

Carb needs to be cleaned Clamp your fuel line Get some jet spray gumout with the tube take fuel bowl screw out of the center of the bowl this is where your high speed jet is Dont loose the fuel jet on the float it hangs there clean it as well the rest of the carb inside put back together should run better

reset the valves to specifications by gaping them all to .008 thousands The vibration will continuely make the gap wider causing this problem to get worst. Gap the valves is recommeded every other year

Generator coil is Shot

Sounds to me as tho it might need a carburetor adjustment, providing there is an adjustment screw on the carb. Sounds like the engine is running a bit lean. Did this problem occur right after the carb cleaning, or was it there in advance? Is there a fuel filter that might be clogged?

you can replace the valves without removing the engine by removing the head first. The valves will come with it, however, the mechanical clunk isn't usually indicative of valve issues. I would remove the head, and push on the piston to see if you threw a rod instead. Sounds like the connecting rod was knocking, then gave way, and now the piston isn't moving at all. Another way to tell, is just to put a screwdriver in the spark plug hole, and turn the engine over by hand and see if the screwdriver moves. If not, you need to tear the engine down and fix your rod issue. If you're lucky, you may find the the end cap bolts came loose, and you can put it back together without too much issue. Otherwise, your engine's probably done.

Dirty air cleaner.

Clogged fuel filter.

Replace or clean both...