Kawasaki bayou 300 starter spins but wont engage all the time

check the battery and wiring first. cos when the starter goes in/out it means the starting voltage has dropped so low for the starter solenoid to keep to keep the high load starter engaged so it goes in/out{ back to basics ohms law} hope this helps

I need to know what type of starter motor yours is

Faulty starter solenoid or relay possibly