I don't know if they've changed it or not but at some point you had to pull the throttle all the way down and then back up, otherwise the engine would stay idle. Hey, that's what you do in real life too, don't you? You don't start any vehicle full throttle!
Another issue I remember having was that I had the RPM on the slider but that affected WEP only in case the slider had slid down a bit.
Speaking of the slider on the stick, I don't know if it's possible to have throttle assigned both to the slider and to the throttle.
And finally, double check that there's nothing assigned to your least used controller, the mouse!