The shiny wire that goes from the carb to the servo, is it hitting anything? If you take off the little L shaped pivot that connects to the end of that wire and the carb (a single bolt holds it onto the engine mount), then you should be able to slide the carb barrel by hand. If you can't, then your carb is gummed up.
The stock TRS (throttle return spring) on the revo's was always too stinking strong for the weak servo they use. Your servo may just be dead. You can try and disconnect that spring (without the truck running) and see if the throttle opens and closes (or at least opens) when you apply the throttle via the remote. If it does with the spring detached, then your servo is on it's last leg.
If you replace the servo, I'd suggest replacing it with something stronger like a futaba S3305. Not a real expensive servo, but it will (or should) be able to overcome the resistance of the TRS better and should last longer since it's a metal geared higher torque servo than what comes on the truck stock. It's actually quite a bit stronger than the steering servos. I ran the S3305 for a long time as my throttle/brake servo on my revo when I had it.
I completely redid the throttle linkage though as I hated that TRS that's on there...
The futaba servo is also compatible with the stock horns as traxxas servos use the same output shaft splines as futaba.
Here's a photo of what I had on mine:
I used this on the end that connected to the L pivot:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD910&P=7
I used 2mm threaded end push rod for the wire. Found it in the wire rack at my LHS. I can find one specificly on tower...
And I used typical
collars/
springs for the servo side.
This worked a lot better than stock. And it didn't require the stock TRS spring as the spring on the rod itself holds the carb closed and allows for more movement for braking. Also, using little
5x8x2.5 bearings in the throttle pivot made for smoother and longer lasting throttle action.
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