I have a guest bathroom that when flushing the bowl used to fill high enough but now the water fills half and even siphons out slowly to lower the water level. I checked all the seals and hardware for possible leaks but nothing. I ajusted the screw on the arm and still nothing. I listend to the bowl to see if I could hear water running out but nothing either. What could this be?

Check to see if the little tube that runs from the ballcock to the overflow tube is attached to the overflow tube and tell us that toilet flush good or not, Thanks.

John

John gives you a good starting point but I am concerned about that siphoning you mention. This would cause me to checking the venting of this drain, I think your vent is clogged.