A replacement carb for my walk behind with the 4HP Briggs Quattro engine was only $18.00. Really. There's no overhaul kit available for it, according to all the places I checked. It's the all-plastic 'Pulsa-Jet' (if memory serves) carb, with no adjustment screws for mixtures. Apparently they don't want ya to rebuild it, just replace it. Mine went for 7 years of pretty heavy use before it wore out and kept running too rich. Popped a new OEM unit on there, pulled the rope, and wow it was like a new machine.
I would have overhauled it if there were a kit available. But for 18 bucks and 10 minutes with the wrench, it was a no-brainer.