Metering rod step-up spring selection - High Vacuum condition

[StockMan]

I have a 68'440 that I rebuilt a few months ago.  The build was all stock with a new MP magnum cam.
I'm happy with the motor, great power and runs nice.  I am running the smaller AVS carb on it however.
I've recenting figured out an annoying 'Surge' I was experiencing while at cruise speeds (50mph or so).
The issued was due to a lean condition that was occurring while at high vacuum conditions. 
The 440 has a consistent 25in/units of vacuum at idle and around 22 units while idling in gear.
I have three sets of step-up springs and I have the stiffest set in there now. 
I elongated each spring by a about 1/4 inch and installed them, and bingo, no more surge.

So the trouble now is that the springs seem to 'Remember' their original length and return there over the coarse of a few days, then the surge is back.
Is there a step-up spring that works well with this kind of vacuum?

[John_Kunkel]

Stretching a coil spring to gain tension is always wrong, get a stiffer spring. Several are available and are color-coded to identify them....trial-and-error.

http://www.carburetor-parts.com/Carter-AFB-Metering-Rod-Springs_p_315.html

[StockMan]

Thanks for your reply.  I've got three out of the four different springs, and I do have the unpainted one which I think is the heavy spring, but none of them work.
They all seem to be setup for the lower vacuum conditions.  I'm not sure why this motor has so much vacuum, firefighter said the smallar carb might be the reason for some additional vacuum.
If I can recall the stock 440 magnum had around 22in of vacuum.   I'm going to buy those springs anyway, maybe a new set will make the difference.