Get the engine running at medium speed or higher. Get someone to sit on the seat, or put a weight on it, or temporarily bypass the seat microswitch. Stand in a safe place where you can see the carburetor's throttle butterfly linkage (from the governor), while simultaneously rotating the high speed mixture screw sticking downward from the middle of the float bowl. See if the governor is hunting - that is, the butterfly linkage is tending to shift around, fairly regularly. This indicates lean mixture.
If it is hunting, turn the mixture screw so as to unscrew it about a quarter of a turn (as if you were loosening the screw). You can probably do this with your fingers. If the engine speeds up a bit, and/or the hunting decreases, you are making progress.
You can also check your adjustment against the B&S recommended starting point for tuning, which for your carburetor is 1.5 turns from fully screwed-in. If you unscrew it too far you will notice the engine slows down a bit and the exhaust starts to smell. The best position will be more or less mid-way between too rich (engine slows down) and too lean (engine slows down), but I have found it best to watch the butterfly movement and try to find the leanest setting where it is completely steady. If it is moving, it's lean.
