float valve is not sealing
on 300's, kawasaki used those STUPID Keihin carbs that didn't have replaceable float valve seats, so when the seat wears all you can do is replace the carburetor with a new one and they ain't cheap
once in a blue moon you can buff the seat with a q-tip and some toothpaste then clean it out perfectly before putting the carb back together. U can't leave ANY of the toofpaste in there or it will stick open and flood the engine quickly
there are some chincanese copies out there for like $80 or so, but they are hit and miss in my experience you might get one that works perfect and the next one (exactly the same) is nowhere near close. Throw it away and buy another until you find one that is close enough to suit your needs
replace the fuel valve on the fuel tank the older stuff isn't compatible with newer oxygenated fuel. Replace the fuel lines too, both of them. There are three on the prairies and they are oddball pre-molded, with one end much larger than the other. I never found anything that "would work" other than the $50 worth of 3 fuel lines from Kawasuki.
lastly if the engine is running rich, that too will build crankcase vapors in the crankcase. So will frequent short trips. Mules have this problem every year from about November through April, I've had discussions (arguements) with customers who said that their 610 had a "blowed head gasket" cause the engine oil was rising and sometimes milky; he'd change it 2 or 3 times in the winters and come June? Problem disappeared....with frequent short trips, normal blow-by gases build up in the crankcase and the oil never gets hot enough to evaporate those vapors out. The cure? On Mules, their governed. Toss it into low gear, floor it and leave it on the floor for about 5 minutes minimum. Does not apply to Pro FXT's, FXR's, any of the Mules with the 3 cylinder Chery engine. Only the Mule 4010 and smaller they are all mechanically governed lawn mower engines that are designed to run 3500 RPM at full load all day long and in a Mule, they just sit and idle most of the time.