mine did the same thing off the line, you could excel really slowly but any quick throttle response was gone when not in neutral due to the weight of the car. It could be a vacuum advance issue, or you may just need to advance the timing 10 degrees-ish.
I got an 85t and i replaced all the belts. I had mine set to 0 degrees and it did the same thing. So i advanced the crank by 10-15 degrees and now it runs better than ever. But try the small things first. Good luck.