Yamaha TTR 230 Dirtbike won't throttle past idle, just starts sputtering and quits?

First of all you take out the plug and use is as a sinker when you go fishing, replace the plug with a new one, and try again. If this does not work, check your float level, and check that the needle and seat is working fine. Your bike is either too rich or to lean, thus getting too much or too little fuel. If you have never touched the float level, chances are good that it is still set at the correct level, just clean and check for cracks in the float.

Trust me there is no problem with the engine, it is your carb and spark.

Have you rechecked the carb? If it will idle ok, it doesn't sound like a fuel blockage. I had a car like that once. It turned out the garage forgot to tighten the carb needle up (SU carb) so it would idle on the pilot jet but not rev. I only found out because I stuck my finger in the carb and the piston wasn't lifting. The only other thing I can think of is, it's getting too much air. Again, could be needle problem.

Yamaha 230 Dirt Bike

If you are absolutely certain the carb is clean, and I mean POSITIVE, (because it sure sounds like a clogged pilot circuit to me) then you need to check your compression. If the rings froze into place over the 3 years it sat, and then you fired it up without popping some WD-40 in there to loosen them up, you're going to have a low compression situation, which might account for your running problems. There's not much left to check with a TTR. Not too many doo-dads and ding-dongs on those bikes...

Well have you tried warming it up, let it idle for 4 minutes. if that doesnt work change the spark plug and air filter and the oil. Your piston could be starting to wear if youve been riding on the rev limiter, their a fairly maintenance free bike but i really dont know why there would be a problem.

Change the spark plug.

check for gremlins, aka, mud dabber's, klogage, stuff like that. O, and like the other guy said, "new spark plug"

check the fuel filter and vacuum lines