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

How do you figure out at what hg your spring operates at?

There are a couple of different ways. Hook up a vacumm guage and then take something like a soda straw and stick it in the vent hole which sits above the piston. Flick the throttle while watching the guage and the straw will rise out of the hole when the spring overcomes the lowered vacumm. Not very accurate though. The way I'm doing it is I have a spare quadrajet (which is dismantled), so I can hook it up along with a guage. There's a small vacumm hole directly below the piston. Then I can start the engine and slowly pinch off the hose going to the manifold vacumm port. The guage will slowly drop and when the vacumm gets to the corresponding spring pressure you can see the power piston slowly start to rise.
Fun stuff

Hmm. What I read is the right spring is the strongest spring that won't over come the vacuum @idle.
I think that's what I read.

Don't forget, normal vacuum is about 20 inches at idle for a stock motor. I'm going to guess Steve is running a fairly agressive camshaft to only have 14 inches at idle. Or, motor has some problems.

Actually, I'm running a new "E" cam. The vacumm guage is rock steady at 14hg or 15hg depending on where my ignition timing is set. But also, I'm at about 5400 feet in elevation. Assuming a vacumm drop of 1 hg per 1000 feet in elevation the 14-15 hg is perfect. Having removed a more agressive cam because of the elevation and switching to a more milder cam, the carb now needs some tuning. I've got the jets and rods close, now time to tackle the spring. Currently I have a spring which is rated at 8hg. That seems mild with my new vacuum readings......

Glad you posted this S. I've got a qj with an APT so the straw trick won't work. I've got a spare carb and a vac pump with a gauge but didn't realize that you cound hook it up to the port to check how much vac it takes to overcome the spring. I'll have to try it one of these days. It's one of the things that has bugged me since the last time I tuned my carb. Thanks for the idea.

Thanks for clearing that up! Makes sense now, I should've added the "at sea level" qualifier.