If it is not running smooth, your linkage is not right, has some slack in it. Replace the linkage from the pvc to the bar attached to the motor with a paint stick or something that you can drill a dozen or so holes in. Add some holes to the pvc, spaced out from the one you drilled. Experiment with it until you get it so it doesn't have dip and slack. Use a cotter pin or a nail, just tape it so it doesn't fall out. you will find a place where it works, transfer those measurements to the bar you're gonna use. You will learn a lot about how this mechanism works and how to find the "sweet" spot, without the math!
Once you have it running smoothly with the foam head, then add the weight and address the counterweight issue, using springs, bungees, or weights, whatever you find works best. Sometimes it is just what we have lying around at the moment.
