Any good rule of thumb on power piston springs. I currently have a spring that starts to move at about 8hg and fully expanded around 5hg. My idle at vacumn is 14hg. I read one place that the spring should be 1/2 the idle vacumn, or 14hg/2= 7hg. If that's the case, then my current spring is about right. The engine is running a little lean right now and along with a possible jet/rod change, I'm wondering if I should go to the next size spring (maybe around 11hg or so)....This is on a 68 rochester quadrajet