I don't know much yet, but will post what I have hoping for some answers. I just dropped by the house for lunch, and need to get back to work, but maybe somebody will understand the problem. I was using my Toro this morning, fired it up, finishing the first bag. It has a 5.5hp 2 cy Sizuki, four season old, but probably only about 1,500 service hours (shares hand mower work with a LawnBoy). The Sizuki has never provided one problem -- always has run well and very efficient. I stopped it to change the bag, and when I went to refire, it oversped. It started on first pull, but was clearly overspeeding. I moved the throttle back and forth a couple of times, but the action made no difference in engine speed. I stopped the engine after a few seconds. I took it to the truck, pulled out my tools -- took off the air cleaner cover and found the choke butterfly was not able to move. It would not move easily with either the throttle to "choke" position, or when I pushed on it manually. I took off the air filter housing to expose the end of the carb. I couldn't see clearly inside the throat, to got my flashlight. On the added light, I could see some obstruction in the throat. It was black, looked like a plastic object, and it was "loose" when I pushed on it with a screwdriver. Using small narrow-nosed pliers, I tired unsuccessfully to pull it out past the butterfly. In moving it a bit further in, the butterfly was able to move freely again, both manually, and with the throttle to "choke" position. At that point, I decided repair was not so simple, so buttoned it back up. I fired the engine again, and the response was the same -- overspeeding. I shut it off quickly and loaded it on the trailer. I had my LawnBoy to finish and to continue for the rest of the day, so I'm not down and out. Clearly, the loose object should not be there. But, without further inspection, I don't know what it might be, and how it got there. After I am done with my mowing rounds this evening, I plan to pull off the carb, but I get a bit edgy with all the connections, tiny springs, etc. Maybe somebody has some experience with something similar and can provide some guidance to help. I'm comfortable with tools, but have not done detailed carb work. Typically, I do all my repair work, but this matter makes me concerned about success. I apologize for the lack of more detailed info, but with the limited observations made while kneeling in somebody's front yard is the best I have now. Maybe I'll learn quickly after I start taking off parts. Thanks for any suggestions, experiences, and observations.