My Yamaha 2400 generator is less than one year old and in trying to use it this weekend it would not idle smoothly with the choke turned off. It seemed like it was reving high then low at regular intervals just like a person stepping on their car gas pedal every second or so...what could this be?

Most likely, the carburetor needs cleaning.
There are two main jets powering your Yamaha, the pilot, or sometimes called the idle jet, and the main jet. If the pilot jet is clogged, the main jet will take on the load, and the governor will modulate the carburetor to keep it running, causing the surging problem. Likely, the problem is less, or even nonexistent if the choke is on, or the generator is under load.
Search the web for help on cleaning this pilot/idle jet.

Take it to a dealer, there is a solid state module inside that controls the charging and idle, also a wiring diagrams are available with owners manual, go to Yamaha web site also..

If may be a partly blocked main jet ( Check by slowly closing the choke), if it the hunting clears out as you slowly close it it prob blocked main or pilot jet...
If the unit hunts (flutter)at 1/4 through 3/4 load, the governor damper spring on the throttle shaft under the stepper motor needs one more turn.. only after you've checker the carby..Only one turn or you will damage the stepper motor....one a few of these....

Did you ever fix the problem?

your carb needs cleaned and fuel line cleaned out

well first try the choke if it runs good when you pull the choke even a little then you need less air, if it runs worse then you need less air, in is less air and out is more air thats the air mixture screw.
idle screw is self explanitory start turning it if it idles up then your turning it up if it idles down then you are idling down usually in is up.

lol buddy if its not idling turn the idle screw in until it idles get back to me if no difference

sound like fuel starved. also drain old gas out of tank in put in some high grade fuel. try starting/running again 5-6 times, should pick up. if not then remove carb bowls, clean. plz rate-bozcro

I'm not quite sure if your issue may relate to most inverter and auto-load sensing generators or some gas-powered equipment.
Most power cutters/grinders such as Stihl or Husqvarna, specially the higher cc models do sound crappy without load, you'd think there's something wrong with it from day 1. This is normal. Well, some of these machines lack the load sensing capability thus does not adjust throttle variably according to load.
Modern generators, specially the inverter ones along the 1000 to 3000 watt range where the Yamaha 2400 falls into category, have a switch where you can choose to let the unit sense and adjust throttle according to load. If this is off, the unit might sound like its not running smoothly on idle. Depending on you, this can be alarming or not. On a Honda EU2000is, for example, it is noticeable when the auto-load sensing button is off but not be like its choking or sputtering.
Anyhow if it is idling roughly, I can only come up with 2 possible causes, if I may suggest:
1.) the cable that gradually pulls the throttle (to open wide or close) on the carburetor - normally called governor cable (although it has different configurations on different engine types) needs to be adjusted/tightened. Or replaced, if worn out. Before new cable, put lithium grease on cable - it helps a lot.
2.) throttle adjustment might be necessary. At most Yamaha servicing dealers, it is included when you have a carburetor tune-up. They will check the diaphragm for blockage or holes, gaskets for leaks and filters, well, the entire integrity of your carburetor. This resolves most rough idling issues.
I know it would probably save you at least $65.00 if you have to check this yourself and buy the spare parts. However, going to a servicing dealer may be the best solution but always be sure the repairs are warratied. Hope this helps.