I just bought a used 1990 kawasaki 300 bayou put a new carb on it guy let it set for about a month now when u turn on fuel it leaks out if the over flow tube what could cause this

Make sure the needle/seat spring is still there. also make sure your fuel filter is clean enough to keep up to your carb's new thirst

well yes it does sound like that so lets find out. The fuel needs to move through a fuel basic things to get used. 1st it must pass through the tank screen and the fuel petcock (that probally has a vacuume sw) to get to the carb,so lets confirm that first. when YOU loosten the carb,float bowl screw does fuel leak out. if YES check the carb.jets for blockage if NO check the fuel petcock for clogs or the vacuum hose attacked to the petcock and be sure its in good condition and connecten to a good vacuum source

Hey Dave,
thought I answered this already,,,??? Classic "brass kit" symptom! >>> You need a "brass kit".. this is the needle jet and the jet needle in the center of the carb. The jet needle is the thin needle that hangs down from the center of the slide... The needle jet is the "hole" that the jet needle slides into...Remove the carb, take the bowl off, remove the main jet, the needle jet holder should be able to be gently tapped out, the needle jet is a short brasspiece,, it comes out through the hole where the slide goes,,, reinstall the NEW needle jet, needle jet holder...(it may have a groove in it for correct location). once its seated, you can reinstall the main jet..NOW >> to install a new jet needle,,, remove the slide from the carb.. remove the spring and seat from the slide that holds the needle jet in place.. NOTE which groove the clip is in on the old needle.... (should be the middle one). re install the new needle, reinstall everything else and your atv WILL run fine for a long time..This is a common problem on ATV's..
you need part #'s
1990 Kawasaki BAYOU 300 4X4 KLF300 C2 Carburetor

http://www.kawasakipartshouse.com/oemparts/p/kawasaki/16009-1503/needle-jet-n36w

http://www.kawasakipartshouse.com/oemparts/p/kawasaki/16017-1270/jet-needle-6

buy a bottle of gasline anti-freeze. it will absorb any water in your fuel. add a half a bottle now and a half next time you fill the tank. should improve the operation of the engine

try playing with the mixture screw on the carbs this pic below will help you buddy get back to me if no difference

ok buddy to stop the back firing on the carb will be a screw turn this in 1/4 of a turn and try this the mixture screw on pic below

No guarantees but try this. Pull the slide from the throat of the carb. Now move the clip on the jet needle down one notch. This will richen the low end mix. Two notches may be needed but do one notch at the start.