Edelbrock 1405 step-up springs, which one?? [Archive] - Chevelle Tech

: Edelbrock 1405 step-up springs, which one??

Mar 22nd, 06, 8:46 PM
Ok heres my dilemma.

I've got a 350 SBC with a cam, headers, edelbrock intake & edelbrock 1405 carb. Im at an altitude of 7000ft.

I've got the timing set good, runs ok still needs fine tuning, starts everytime. The cam specs are unkown.

Idle set at 900, 8" of vacuum, fluctuates within a 1/2 because of the cam. Idle mixture screws set using vacuum gauge so im good there.

All is well except i dont know if i should change the step-up springs on the carb or not considering how much vacuum i get. And if so, which color should i use??

I've read a lot of articles on which to use, but am not sure which because of the altitude im at, the cam & amount of vacuum it pulls.


Mar 22nd, 06, 9:20 PM
Off the top of my head I don't know what the stock springs for your carb are. They are calibrated against inches of vacuum. You want a spring that will ensure the rods are down at idle, where you experience 8" of vacuum. So pick a spring with at least 8'' and even a little lower less.

Step up springs will bring about a very subtle change in driveability.
Stiffer springs will "come in" quicker, lighter ones later. I suggest trying them, one "step" (no pun intended) at a time and then drive the car several miles with lots of stops/starts feeling for differences.

Since they are so easy to change, experiment a bit until you understand what your engine responds to the best.

Mar 22nd, 06, 9:42 PM
Ok so i just got to try different spring rates and see which drives best. With that in mind what kind of drivability differences am i looking for??


Mar 22nd, 06, 9:59 PM
Drive around making part throttle changes, don't stomp on it and go to WOT -- the idea here is make the vacuum drop a little bit but not go all the way to zero. As the amount of engine vacuum crosses the point the spring is calibrated at, the rods will move from one step to the other and change the mix.

If you come into the power step too late (i.e. spring too weak) you may get a little flat spot or stumble. Too early and you may go rich too quick and get a little bog. It'll be really subtle, less than what a rod/jet change would get you because you are not changing the mixture amount only the timing of it. You may not even feel it, which is why I say drive around a bit to be sure.

It could be, if you can't feel a change in the seat of the pants driving, that the impact would be mostly mileage.

nick v
Mar 23rd, 06, 12:50 AM
I think the orange springs are stock in the 1405, 1406, and 1407.

Mar 23rd, 06, 10:08 AM
I'd try the pink springs and see what happens.

Mar 23rd, 06, 1:48 PM
These springs are the last thing you should try and tune. In many cases you will not be able to tell a difference. Edelbrock only recommends changing them to address "driveability" issues.

I don't know where this idea comes from, but these springs have absolutely no relationship to idle vacuum anymore than a power valve in a Holley does. Idle runs entirely off of the idle circuit and these rods affect only the primary main circuit. The idle circuit may actually draw fuel from the same well as the primary circuit, but the fuel restrictions (read: jet sizes) are radically different with the main being much much larger than the idle no matter what rod step you are on.

If you hooked up a wide band O2 AFR meter what you would see is that as vacuum progressively drops so does the amount of fuel. Thus the engine goes leaner. At some point you hit the needle/spring threshold and you get an extra amount of fuel. Thus you might think of the AFR as a couple of sawtooth notches. You want to tune such that the bottom of the notches stay at 14.7 or above. For best economy you want to jet/rod to get as close to that 14.7 as you can. More fuel than that at cruise is just a waste.

Personally I have a theory that these springs actually compensate for the idle/transition circuit turning on/off. When the manifold vacuum drops to a point around 5-7" the Edelbrock idle/transition circuit shuts off. This is by design and controlled by the relative sizes of the fuel restriction and air bleeds. At this point you need extra fuel from the primary main circuit to compensate. Note that this has nothing to do with the motor and it's vacuum level, it is all about the Edelbrock idle circuit air bleed sizes.

Edelbrock technical documents claim the piston/rod compensates for load. Maybe, but if I really want to load the engine I am going to open the secondaries. So that is where I want to put my major extra load compensation fuel and drive the AFR towards 12.5.

Also it seems to me that if you idle at a fairly high RPM and at only 8" then either you have a serious cam in that motor or your ignition is not setup correctly at idle. Maybe a combination of both. My 402 with an HR296 used to idle at 850 with around that vacuum level, until I put in the right vacuum advance and hooked it up to manifold vacuum. Now it idles around 11" with 30* advance at idle, 18* basic initial plus 12* vacuum advance.

Mar 23rd, 06, 10:12 PM
vrooom3440 - I wish i knew the cam specs to my 350 but i bought the chevelle like that. Closest comparison i can come to is if you've heard what an LT4 Hotcam idles like, very similar. I know that still doesnt tell much.

If i go higher on timing, engine vacuum goes up as well between 10-11". So this saturday i was going to raise timing more and fine tune it again. I know i can get more vacuum out of it, i just dont like the idea of going so high on timing . . but its worth a try.

Thanks for all the info, it really helps me as im new to carbs an such :)


Mar 24th, 06, 1:35 AM
What is your initial timing at the moment?

I went through this myself and it was quite a learning experience. Note that while I idle at 30* I startup at 18*. Nothing fancy on the starter and never any problem cranking it over. Actually starts as well as my EFI Mustang so long as it has not been sitting for more than a couple of days.

I am running a 22* 3-9" vacuum advance can that has been limited to 12* so it is more like a 6-9" vacuum advance can. Thus it kicks in the extra advance I need for idle once the engine is running.

If you are relying on initial timing to set your timing you are smart to be wary. You really want to set your timing on total without vacuum advance to 36*. That is the critical parameter and after that just see where your initial falls. If it falls too low you may then need to reduce the amount of advance in the mechanical advance. But note that the vacuum advance setup right can help out greatly here. Just do not have more than about 12* of vacuum advance, you want to be around 50* at cruise with full mechanical and full vacuum kicked in.

You really need to get the ignition right before you do much with the carb.

Mar 24th, 06, 8:59 PM
I should have been more clear about my comments relative to idle vacuum, as Vrooom is correct. My comments were intended to ensure that the rods where down or on the lean step at an idle condition so that as you transition to main metering, as vacuum further drops off with throttle opening then the rods still could move up and richen the main circuit. If the rods are up at idle, although your not idling on this circuit, there is no "room" to richen this circuit later if you need it.

As mentioned, major transitional changes cannot/should not be tuned with the springs.

I thought you wanted to play around a bit based on your "all's well" comment. But if this isn't quite the case, get everything else dialed in first.

Vrooom, I like your working theory, I've got to think about that one more to really "get it". When it comes to Edelbrock's comments about springs reacting to load, I think of this as what happens when you encounter a long gentle hill -- not much change in throttle position, operation is pretty close to steady state conditions. I'v come to this idea because Carter had some three step rods on motor home carbs in the 70's -- an application that has a fairly healthy throttle opening at mid-RPM steady state cruise conditions -- they'd run on the middle step. Without it, they'd be on the power circuit.