B & S 3.5 on a Murray push... ruff running... approx 4 yrs old.
Just started running bad last month. Ran perfect before with no issues.

The manual shows how to adjust carb and throttle.

But mine has no throttle... it starts and runs at one speed.
But now it misses or sputters.

I changed spark plug and airfilter.

Check for contaiminants in the fuel, or perhaps check the carb diaphram if equipped with that type of carb.