Three servos, all rated for 6 volts and all three cooked. It's got to be binding.

A short, stiff, 4-40 pushrod is what you want. With a control surface other than a throttle, there is usually some resistance that helps you see when you are approaching max deflection (if you are even going to maz deflection in the first place). But when a carb bottoms out it bottoms out. It absolutely stops against idle (unless you remove the screw) and it stops against full throttle. No slop.

I'm fairly certain that the servo is banging against stops at idle, full throttle or both and it could be in a very small amount.

You could try disconnecting the purshrod, manually push the throttle to full and then check to see what kind of alignment you get when the receiver pushes the pushrod to full throttle. The travel can be too long or short by the diameter of the pin in the clevis...it doesn't take much. Check at idle too the same way. Also make sure you have the necessary wiggle room when you move the throttle trim tab as well.

Hope that helps some and please let us know how you make out!

Tom