Fix a running toilet

Troubleshoot Your Toilet Without a Plumber

www.popularmechanics.com/home/how-to/a4117/4303948/

Overflowing, constantly running and sweating toilets can test a home owner's sanity. ... take for granted--until we can't take a shower or make the toilet flush properly. ... If none of the above does the trick, you may have a misadjusted or faulty fill ... the waterlevel a half inch below the top of the overflow tube inside the tank." ...

Emergencies - Overflowing toilets - Toiletology 101

www.toiletology.com/emergenc.shtml

You can't just lower the water level in the tank; you need to displace some of ... and the ballcock and flapper are working properly and shut off when the tank is full. ... The bowl is filled with the water that goes into the overflow pipe. ... Unfortunately, when a toilet bowl is clogged and the water level rises and doesn't flush the ...

replace the flapper

if the water keeps flowing then try replacing the flapper if it still flows then the fill tube gasket or tank to bowl gasket need to be replaced.

Check to make sure that the bowl fill tube is in the overflow tube. Your description sounds like the bowl starts with less water than it should have, so it has to fill up before actually being pushed out.

flapper is leaking or worn letting water run thru

The only adjustment available is to control how full the tank will be for the next flush. The water that runs down into the bowl after flushing comes through the same valve that fills the tank, so the problem (and solution) is entirely with the amount of water that runs into the bowl while the tank is filling. The tank is filling sooner than enough replacement water can run into the bowl. Make sure that the tube (usually a 1/2 inch rubber tube) that runs from the float valve to the pipe that fills the bowl (usually a 1 inch pipe with an open top) is actually delivering all of it's flow down that pipe. If the flow of water being delivered by that tube is slow, you will have to find and eliminate the restriction. You might be able to pull of the rubbber tube off of the float valve and stick a small drill bit (just using your fingers, not a drill) into the fitting where you pulled off the tube.

this sounds like a leaking flapper.although you may not see a leak, this is always the problem.a leaking flapper can be replaced,just becauce the toilet is new does not mean that it may not have defective parts.i would check the flapper first then the fill valve.this will most likely fix your problem.good luck with your project.

Check the fill tube in your toilet one easy way to see if it's leaking is get food coloring and pour in your tank to color the water this makes it easier to see and squeeze the tube in the tank when it's full if you see the colored water running down the middle of it you have a winner! if not you can look around for it sometimes the plastic under the flapper gets cracked too

First off,If the air vent was clogged the toilet would back up and overflow and when it did flush, it would make a gurgling sound.
secondly the water level in the bowl is not important as long as it covers the opening,The water is only there to keep out sewer gases so they don't enter your home and the shape of the trap will always maintain the proper amount of water needed to prevent this.
The previous water level may have been due to a partial clog that has since been removed.