On the revo, the TRS pulls the carb closed. With the engine off and throttle trim at 0, there should be a bit of a gap on the throttle slide before the slider makes contact with the collar and moves it. If there isn't a gap, then you need to move the collar.
You can see the gap here between the yellow slider and the silver collar:
Obviously, this isn't stock, but stock should have a similar setup.
I didn't like the stock setup relying on the TRS and requiring such a stiff TRS, so I used a softer spring, but added an additional spring/collar like most other RC's have so that when I was at "idle" on the transmitter, it pulled against the throttle a little bit and compressed the spring on the throttle wire. When I applied brake, it would compress the spring more. But the spring wasn't overly stiff and there was still a softer throttle return spring connected to the carb if I lost power.
Anyway, with no gap, it would suggest your transmitter isn't at 0 for trim, or it is and the collar needs moved a little. This allows for the TRS to pull the carb fully closed against the idle set screw. Then that screw will adjust your idle. If there isn't a gap between the collar and slider, adjusting the idle screw won't help because the servo will be holding the carb open.