Still could be a number of different problems, but likely a carb/fuel issue yet. You may have swapped one problem carb for another, albeit with the same symptoms.
Background: On the MFS8 and 9.8, the idle/slow circuits are very high velocity to maintain good fuel mix at super-lean mixtures (thank the EPA). That means that they are very small, and very succeptible to both varnishing and debris. The varnishing occurs as fuel evaporates out of the carb when it's not running. Over 90% of the time, we are able to clean varnished carbs by soaking in carb dip. That condition is typically due to not running the carb out at the end of the day. The exception to that is if there is oxidation (due to water contamination), exacerbated by ethanol blends -- thank the EPA again, leading to a scrap carb, which cannot be cleaned by ordinary means. So your original carb, if it was not oxidixed, probably could have been successfully cleaned, after complete disassembly and a 4-hour soak in real carb dip such as Tyme.
As for the new carb, assuming that you didn't leave fuel sitting in it for a week (causing a recurrence of varnishing), there is a slight possibility that there is a microscopic bit of debris in it, that did not affect running during the first day. Sometimes that debris is a tiny fragment of metal or rubber/plastic left over from the manufacturing processes. I have personally come across a couple of carbs (from various manufacturers) that have had that sort of problem. In one case, a microscopic bit of metal was loose in the passages, and sometimes caused no problems at all -- yet at other times would restrict the idle passages. The motor passed tank test, the customer was happy, and a month later, started getting idle problems. Took a few disassemble/inspect/reassemble/test cycles to find that one.
I would consider a methodical and thorough/complete disassembly/inspection of the new carb. You will need to pull out the jets and emulsion tube ("main nozzle") and the top cover. You will want the brightest possible shop lighting (or bright outdoor sunshine), and maybe a magnifying glass. Don't forget to reinstall the rubber "cork" in the jet well when you put it back together, or the mixture will be so rich that the motor may not run at all. Also verify float height while you have the bowl off.
