Just an FYI more than anything. I have an ST 224, 2015 model, and it was surging. I thought I just needed to adjust the carb but you can hardly see the carb much less than adjust it. So I started disassembly and got to the carb, found there were no adjustments to be had, only the idle. I removed the carb, disassembled it, used carb cleaner, blew it all out. There are youtube videos that show this. Put it all back together and it runs great now. For some reason, on this particular carburetor they decided to put fixed jets so there are no adjustments. If you have a rough running or surging engine, looks like the only way to fix it is clean the carb. They do suggest after each use, close the fuel valve and run it till it stops. After the season for storage do the same then empty the carburetor bowl of fuel. I hope this helps someone maybe save some time and frustration.
After get the carb cleaned / running issue resolved I suggest adding seafoam to your fuel can and use premium fuel, anything older than 30 days old pour into your card tank......never any trouble from my mowers or blowers doing this (or cars). Good Luck
NWI Busted Knuckle (Garage)
Installing ArmorSkids has been the best mod made to any of my machines