If I rev the engine in my 85, once the rpms drop down below 4k, it sputters all the way down to idle. Once it's over 4k, decelerating the engine from anything higher than 4k down to 4k is smooth. What do you think it is? Fast-idle thing, anti-afterburn, shutter valve....Thanks

 

Sounds like a bad shutter valve to me. The shutter valve's purpose is to stop fuel from spraying during decel, which would explain it. Or maybe that's the anti-afterburn, either way.

 

How do I test it?

 

Don't I have to clamp down on the hoses to test the vacuum?

 

Clamp the big hose going to the shutter valve with a small pair of vise grips. Make sure they're secure and run the car for a bit. That'll tell ya.