[A Snake]

Just for grins, try getting the motor warmed up, get it to hang up, turn off engine and go look at every bit of the linkage closely. The screw hang up sounds like a good bet to me. Also consider a very slightly bent throttle rod (goes through the base and has the butterflies on it.)

Last, look at the angle you have the return springs on compared to the direction of travel. Don't be afraid to put a pretty healthy spring on. I've "Solved" several "hang-ups" this way. couldn't find the hang up, so I put on a bigger spring and voila!

Al