Edelbrock springs 1406 and 1405

Bewy said

I have to disagree with RRR about the method of choosing the correct step up springs. You need to swivel the piston tops so you can see the pistons. Use the strongest springs that will just prevent the pistons from moving [ lifting up ], engine idling, in gear. Once these are known, you can try lighter springs but you might get a bog because it is too slow to enrichen the mixture. Prolly only do this for economy, but I doubt that is a worry with two carbs...

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Then you will be disagreeing with both Holley and Edelbrock, as they both say to cut the manifold reading in half to make the spring choice, just like the power valve. Although I use cruise vacuum with the Holleys, both say that the metering rod carburetors are very difficult to get a good cruise vacuum reading on and I have verified that on the vacuum gauge. The LEAST little difference in the gas pedal or least little incline makes a big difference in the vacuum reading, but the Holley power valve is easier to use cruise vacuum. I JUST got off the phone with the Holley tech line this morning discussing this as I am tuning on my 625 Street Demon. They said word for word to use idle vacuum and cut the direct reading in HALF to make your choice for springs. Since Edelbrock uses the exact same springs, their method is the same and I have verified that with Edelbrock as well, trying to tune this 1407 750. The Street Demon however, is head and shoulders better.