yardman string trimmer idle problem

I have a yardman YM1500 gas string trimmer which is refusing to idle. The engine starts, but as soon as I move the throttle position lever from the "start" position to the "run" position the engine dies. I've drained the tank and filled it with freshly mixed fuel, cleaned the air filter, checked (but didn't change) the spark plug, cleaned the exhaust, and even tried turning the idle adjust screw a quarter turn, but it still refuses to idle. Can anyone provide suggestions for what sort of adjustments or repair the unit might need? Even if it isn't something that I can tackle myself, the nearest MTD repair facility is a ways away so I'd like to have an idea of what's wrong before heading down there.
The trimmer is about two years old, and just out of the warranty period, natch.

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