edelbrock step up springs

[Harlow]

Is there a general rule for what rating step up spring I should use? Or should I just experiment? I get 9 inches vacuum at idle.

[Ghoste]

It is mainly a trial and error procedure.  You will need to hook a vacuum guage up and know what the engine is at under a variety of conditions.  Think of changing the spring in the same sense as changing the power valve in a Holley.  Too heavy and the spring will overcome idle vacuum and give you too much fuel at idle.  Too light and it may never open giving you a lean mixture when you want it fatter.

[Chryco Psycho]

the rods need to stay down at idle in gear , I usually go 1-2 " under the vacuum reading at idle in gear

[Harlow]

I get about 9" at idle. It has the stock 5" springs in now. Should I try a 7" and see what happens?

[Ghoste]

What do you get for vacuum at a cruise rpm?

[Chryco Psycho]

9" at idle IN GEAR ? or is it a 4spd?
thew 5" springs are fine

[Harlow]

Thats with the car in park, I forgot to check what kind of vacuum I get with the car in gear since I changed that carb and got it dialed in. I'll check it next time I fire it up and see what I've got. I'm guessing that with a 100 rpm drop I'll drop to 6-7"?

[Ghoste]

Going up to the 7 would be to eliminate a lean spot if you're opening up from a part throttle position.  Not slamming it open because then you're going into the whole WOT acceleration mode but a stumble if you are gradually opening it up.  That was why I wondered what the cruise vacuum was.  The rating on the spring means that it will hold the rod into the jet until it hits that vacuum point.  Changing the spring changes the point where it allows the rod to come and lets the carb run a little richer so unless there are spots where you find it is too lean or too rich, the springs don't need to change much.