Did you adjust the float to compensate for the angle it's mounted at? What main jet size are you running? What differences have you encountered when moving the metering jet up a couple of notches?
There are many people both past and present on this forum who have had great luck with Mikunis, and Chikunis, a name we call the Chinese versions. They work especially well on the Briggs engines. They are not a plug and play carburetor.
You are going lean. You have too much air flow and not enough fuel. If all things are correct like no leaks and float adjustment, ie it has fuel, then your main jet is too small, and/or your metering needle sits too low in the main jet not allowing enough fuel to flow.