I found that the stock spring on my 63 was all that I needed. Also, get some graphite powder lube that you use for locks, etc... and lube all of the pivot ball/sockets. I (*may have* - I don't remember if I got this to work) took the outer carbs, removed the throttle plates and tightened the return spring by rotating the shaft one turn so that they have a little bit more force. I know I was messing around with that but don't recall if it worked or not. Orignially, I had all sorts of springs to keep everything closed and ended up with a lot of binding and not returning to idle after a hard romp on the pedal.