I have a generac portables power washer with a 4hp briggs stratton quattro. The only throttle adjustment is rabbit to turtle. The rpm of the motor spits and sputters. Very rough. I cant see any adjustments. Are there adjustments ? Is this a lost cause ?

I have changed the oil, spark plug(.30 gap)and cleaned the air filter.

no, not a lost cause.....first off, does the controls actually move anything on the carb? it may have lost some of its linkage, if anything was done to it....now if you've always had it, and its just started it. was it stored right? make sure no water is in the gas, the gas is not old/stale and then move on to a probable gummed up carb if it wasn't stored right.