As tabetha has said, usual cause is running lean. A YB engine needs to maintain at least 3%CO when cold to prevent hunting, (I actually map my engines to start at 7%CO for cold start ramping down as the engine warms up.

Check your base idle when warm is at least 900rpm with idle valve disconnected, and check the idle CO for both cold start and warm running. Perhaps you ECT sensor is faulty!