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I have circled in red, the only mixture adjustment you appear to have (unless there is one on the other side of the carburetor). I'm pretty sure it is just idle mixture though, because it is located at the throttle butterfly rather than at the venturi. You could check whether it has any effect, both at idle and at full speed, to verify this. If it is just idle mixture and your rich-running problem is at full speed, you need to look at Plan B, in my previous post.

You have said it is running rich, and we haven't discussed what makes you think so. Can you describe the symptoms please? For example, if it is blowing sooty black smoke, it is running very rich. If it is blowing a short black puff on start-up it may be nothing at all, or a governor issue. If it is blowing bluish white or blue smoke, that is oil and might have some very simple cause -in particular, on a Tecumseh engine, you might have put the crankcase ventilation valve in upside-down, which happens a lot on those engines. (The little drain hole in the valve housing has to go at the bottom, or oil will pass through the breather into the engine.)