I've got a 30 year old Snapper/Tecumseh mower that my Dad bought in 1983 and gave to me about 10 years ago. I've replaced all the parts around the engine and driveline, but never opened her up or removed/cleaned the carb or used any gas treatments. Still starts on the 1st pull all summer long. :shrug:
I'll give Stabil a shot though. I bet the 40, the KTM and the mower would all like some.