My 2003 yz250 runs full throttle as soon as it is started. Checked the obvious stuff - carb throttle....it started revving out of control intermitantly now its full time. Any ideas?

if your saying your saw runs full speed as soon as it starts , one of two things is happening :
(1) your low speed carb jet is too far open
(2) your throttle trigger has disconnected or become stuck from your carburetor and now the throttle blade inside carb is full open

depending on the machine it is, this could be the AIR/FUEL screw it too lean.
Often it is the AP (accelerator pump) on the carb. This has diaphragm and a pin. If this gets dirt in it it can cause erratic very fast idle, once revved or "bliped". Simply remove carb. On the bottom is the bowl, and another cover. Remove the bowl and this other cover and delicately cleam both. If diaphragm is dirty chances are bowl full of gunk too.

sounds like a carb overhaul time as well as a fuel system flush

after getting the carbs done..did you get them resynicurised..could have a float not adjusted correctlly or a adjustment screw not dialed in correctlly,all 4 carbs need to be adjusted insynic..if not will make for a hard start or run too lean or rich

check valve clearances, and low voltage can cause backfire, battery voltage should rise when revved, use a digital multy meter

Try this:
Clean and gap the spark plugs. Clean the air filter,and then run some carb cleaner through the carb.
Hope this helps.

Clean the carbs. Then move the jet needle clips down one notch. This should solve the dieing problem. Chances are the engine is flooding due to twisting the throttle too much. Try it with only one twist then no additional throttle. Install a fresh set of spark plugs.