Are you sure the T is not maybe an I instead?
In any case, I suspect the T would be the Idle Stop adjustment, which basically just holds the throttle open to keep the engine running at Idle, screwing it in (clockwise) should increase the idle speed, while unscrewing (counter clockwise) will decrease the idle speed.
The low speed adjustment is a mixture adjustment, screwing it in (clockwise) leans out the mixture (less fuel) and unscrewing (counter clockwise) enriches the mixture (more fuel). The same holds true for the high speed adjustment screw.
The high speed is adjusted for high speed operation (cutting with full or partial throttle) and the low speed is adjusted for low speed (idle) operation.
Since your engine want's to die at idle, you may not be getting enough fuel at low speeds, you may want to turn the low speed screw out (counterclockwise) a 1/4 turn or so to see if that helps. It's also possible that your carburetor may need service and possibly new diaphragms to make it operate properly.