Tip to verify that power piston spring is not to stiff

[73 Z28]

Saw this tip on the net and thought I'd share it. To check the power piston spring with the engine running at idle and in gear(get someone to help or set parking brake for this) take a straw and slide it down the vent tube on the carb. (Note: this will only work on q-jets where the power piston is directly under the vent tube)
If the power piston spring is to strong vs. engine vacuum, the power piston will bounce up and down raising and lowering the straw erratically with the engine idling in gear.

[jamesF]

So this process reveals a "too strong" spring by allowing you to see the movement by way of the straw  moving?  Interesting! Should the spring NOT be moving if it is the correct resistance?

[73 Z28]

 With the engine idling and the correct spring for the engine vacuum, the power piston should stay down, only rising if the throttle is opened to reduce engine vacuum. This should be checked with the engine fully warmed up by running approx. 10 minutes to make sure that the choke is off.