You have found an air leak at the throttle shaft, and this leak no doubt contributes to the lean mixture. Since your engine is 5 years old, there is very likely some gum/varnish deposition in the main jet, and in other parts of the fuel system. If you add "carburetor cleaner," or "fuel injector cleaner" to the gas tank, and then mow the lawn several times, over a month or so, there is a good chance that the misfire will resolve without any mechanical repair to the carburetor.

Thanks for the reply. I added some Gumout to the gas and will see how that works
Robert

Obviously, a worn t shaft/ carb boss will have an affect on runability. (The shaft is serviceable BTW. It is all subjective to how much wear. Many of these still fail the "Gumout Test" at the throttle shaft and still run good enough. What may be helpful, is to remove the idle screw/ jet from the side of the carb and spray your jet spray in the passage as the engine is running, reinstall the jet and see if it smoothes out. Go ahead and spray down the carb throat too to help releave the valves of any deposits from old fuel etc. I've found this helpful many times. There are also one or two "o" rings around the main nozzle up in the stem of the carb to replace. My own take on carb cleaners such as Gumout or Seafoam (preferred) is to use it for a mowing, maybe let it sit overnight and re run the engine with tank topped off with fuel to keep it from laying in the carb and affecting seals in the carb. Used as directed the stuff is fine, but sometimes folks get over zealous with the stuff.

Pull the brass bolt from the bottom of the fuel bowl and clean all passages/orifices in it, particularly the small one at the top of the threads. Use the soft steel wire out of a wire tie after stripping the paper off, then the carb spray again.

"There are also one or two "o" rings around the main nozzle up in the stem of the carb to replace."

What is a source for the o-rings. The kit I bought did not contain any. maybe I need a more complete kit...

I do not see an idle/jet screw, is it behind a seal plug on this carb?

Thanks
Robert

The easiest way for you to see, is to visit toro.com and use their parts viewer for the correct model mower you have. The screw is behind a small cap which can be snuck out w/ a small screwdriver. Don't worry about damaging it. The rings are available separately. If the nozzle stayed in place and you didn't have to dig out black from inside the main jet they may be fine.

Here's a link to the parts viewer. Type in your mower's model number to see a list of assembly drawings.

Here is a link that might be useful: Toro parts viewer

Thanks guys. I have to go out of town for a few days. Once I resolve the problem Ill post the results.
Robert