[theblurrrr]

I'm in the process of dialing in a 67 2+2 428 with a 7027262 carb. The engine, just rebuilt, is running a Butler "067" cam; the engine is now closer to a 433cid. Otherwise all stock. Now running about 10.9:1 CR.
I'm running rich at cruise with a "loading up" condition at idle. I'm thinking I might have a power piston spring mismatch. I know I should I should have "70" jets and .041 rods(which are impossible to find!). The parts book says I should have a 7002071 PP spring.
Has there ever been a post rating the original PP springs with a relative vacuum release spec? Is there some way to determine the spec of whatever PP springs I may encounter? How many variations are there of the early "postless" power pistons and how would I recognize them?
Thanks in advance!

[25stevem]

How much vacuum are you pulling at idle and what’s your idle rpm?

[Half-Inch Stud]

I think this is a good question; spring heights differ short vs normal. Tension might vary in wire diameter (. I forget ).

Some relief is found in knowing when the piston is down ( hi vac ) like small cam idle and light pedal cruise.

Relief is needed for needing to know when the piston needs to be up, and that is any time the foot is pressing the pedal to lightly accelerate.

And of course the piston is up when heavy footed.

So, i use the normal length spring and hoped for the best with the normal combos. As for my over-cammed low-compression combo, no luck, even with Starting, nor idling which shows my engine needs air bleeds modified, and the Primary piston spring is a minor detail.

I can check for a set of 70 and 41.

If Primary side Loading-up the could be leaning-out like a daft Accell Pump (try inner rod hole). 0, 20, 30 mph pull-away evals are good to dial-in the Pri accel pump and Secondary air plates, but this assumes the Dizzy advance and rotor phasing is properly set.
How the dizzy initial and total adv looking?

If problem is Secondary stumble then air damper plate tension needs a look. Vac dasgpot & linkage needs checked too.

[gto4ben]

theblurrrr, if you're asking how to calculate power piston start and WOT positions as a function of manifold vacuum, I've created this table for various power piston springs I had. Measurements were made using a modified '67 fuel bowl where I can apply a known vacuum to the PP bore and determine the upper and lower heights. Piston to bore leakage exists so there is some error compared with the calculated for ambient conditions. Compare your PP spring dimensions with those in the table to determine its behavior.

I have two original '67 7037262 quadrajets and one PP spring was deformed. I was trying to determine a exact replacement which caused me to make this table.

Hope it can help.

[Shiny]

Originally Posted by gto4ben

.... I've created this table for various power piston springs I had. Measurements were made using a modified '67 fuel bowl where I can apply a known vacuum to the PP bore and determine the upper and lower heights...

Thank you for sharing this. It reflects a lot of parts, work, and commitment to compile. The "expected" vs actual along with the design parameters makes your table a very valuable tool.

Mike