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Stumble on secondaries

17K views 140 replies 21 participants last post by  Tegguy 
#1 ·
Ok I have tried searching but I haven't had much luck (maybe wrong search words) I have a 396 with an edelbroc carb (unsure of the size right now but I wanna say 600 or 650 CFM). The car runs great at idle and cruise but if I give it a lot of throttle (basically anything where the secondaries are activated) then the car stumbles and pops a little bit. If I give it a little gas back off then get back into it she'll go pretty good.

Also if it's been running and I shut it off and wait like 5 minutes it won't start unless I hold the gas pedal all the way down. I am unsure if these are related or not.
 
#2 ·
slow down the secondary opening until the stumble goes away. then play with the accelerator pump shot and the door opening until you find that magical spot where the car just pulls and pulls and pulls without you ever noticing the secondaries opening.

and that's just the carb- you also need to play with the ignition system to really dial it in, as well. figure out how much initial timing it wants, how much total timing, and how much vacuum advance- and whether it wants it ported or (more likely) manifold vacuum... it's a whole system and everything feeds off everything else.
 
#5 · (Edited)
#6 ·
I think that you may have a float level issue to start with and possibly a power piston setting problem.As far as when they start to pull the rods out of the main jets under load. the secondaries have a weighted air-valve that controls the opening rate But before you dive in as been said,get the ignition/timing dialed in before looking at going to deep with the carb.Get the edelbrock tuning guide for the carb so you will know your baseline settings.If the engine is a mild set-up it shouldn't take much to get it right.
 
#7 ·
yeah, get the Edelbrock tuning manual that they included with the carb when it was new... it will help you to get the carb back to where it was when it left their factory, then help you tune the carb and ignition to perfection.

i'm pretty sure they'd send you one for free if you call them, and maybe they have it as a downloadable file on their website.

(edit) here you go, the result of 3 seconds of typing "edelbrock 1406 tuning manual" into the google search bar in my browser and a couple of clicks:

http://www.edelbrock.com/automotive/misc/tech-center/install/1000/1404_manual.pdf
 
#8 ·
I've read the manual and have been told the timing is good by the shop I paid to do it. I've read the tuning manual but I suck with carbs so I was hoping it was quick and easy.

I think everything is close and it's just going lean with the secondaries since if I blip the throttle first it'll run great. But I could be completely wrong too....
 
#18 ·
the manual walks you thru every step, makes it easy if you are looking at the carb while reading it.. you need to either find a car guy that knows what the hell he is doing or get some basic tools (screwdriver, timing light, distributor wrench, vacuum gauge) and get your hands dirty. and with a carb and timing, you shouldn't get your hands very dirty.

this is the tuning part of owning one of these cars- simultaneously the most frustrating and most rewarding part. it will frustrate the hell out of you right up until you get everything just perfect with that one last small turn of a screwdriver, and it will be the most rewarding thing you have ever done to a car. when you drive it, you will know that you made it run the way it did and you will learn to make it run even better and pull even harder over time.
 
#9 ·
Had that carb on the 350 in the 66 for seven years.

Read up on the instructions. Set the accel pump in the hole to make it most sensitive, so that it pumps as soon as you push the throttle.

Then perhaps try some smaller tipped rods in the jets you have. See if that makes an improvement. If so go again. Maybe go richer on jets and rods, see what effect that has.

From reading, the 1406 may be leaner as produced than a 1405.

Also make sure you do not have any vacuum leaks from cracked vac caps, etc. One of those gave me a fit for a week until I found it.
 
#10 ·
>>> "have been told the timing is good by the shop I paid to do it"

that type of resonse usually means they have no idea. Ask them:

1. what is the initial timing, no vacuum to the vacuum advance

2. What is the total mechanical timing (no vacuum)

3. at what RPM is the timing advance complete?

4. What is the initial timing with vacuum advance hooked up?

5. what is the total timing with vacuum advance? at what RPM?

6. is the vacuum advance hooked up to manifold or ported vacuum?

"timing is good" is meaningless.

if you have a bog opening the secondaries you might not be to tune it out. I've dumped more than one of these carbs because of this. Didn't matter what rods, springs, float level, jets, etc, etc. I don't recommend these for just that reason.

BTW, when asking those questions don't mention any numbers at all. Pretty good chance they'll tell you whatever they think you want to hear. Make them tell you or show you with a timing light.

If they give you any static about these simple questions don't take your car anymore. For an experienced guy this is 5 mins worth of work which you already paid for once.
 
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#12 ·
The advice you are getting on knowing and setting your timing is right on. It's quite possible the shop set it to stock settings and if you have any sort of performance cam, it's going to need more timing.

Edelbrock tuning starts with checking the float height. Don't assume this is correct right out of the box. The 1406 does not have an adjustment for the secondary air door.

The accelerator pump linkage holes determine the timing of the accelerator pump shot. IIRC you carb was set from the factory on the middle hole and you can try moving it to the hole closer to the carb to deliver the pump shot sooner.

Likely that you need to change rods, and maybe jets. The Edelbrock manual has charts showing which rods to use in roughly 4% increments on the cruise and power circuits. The rod and jet kit also comes with step up springs. You can try going with stiffer springs, which will put you into the power mode of the rods at a higher engine vacuum (i.e. sooner).
 
#15 ·
After timing and float check, determine if your problem is with the power mode or secondary. You can disable the secondary by disconnecting the linkage on the passenger side of the carb. When I did this, I found the stumble was still present and could tune the power mode circuit with spring and metering rod change.

What happens when you go from cruise to WOT? Mine ran fine in that case, also indicating a problem with power mode (partial throttle opening from cruise).

If you disconnect the secondary link, use a wire tie on its actuator to keep it closed.
 
#16 ·
On this carb I don't think you can disconnect the linkage.

The problem that I'm experiencing I've only tried from cruise. For example traveling at 40 and go full throttle she'll just fall and sputter. I have not tried wot from a full stop acceleration
 
#19 ·
I bought a complete Edelbrock jet, needle and spring kit and went at tuning my 1405 on my HO350. As received, it ran..albeit poorly. Over time, back and forth with different combinations, I got it to the point that it really runs well. I might add, it was one of the most satisfying things I'd done with my SB, it's something anyone can learn, and the Eddy guide is a good one. Of course, a large part of the good result was acheived with playing with the ignition, in of experimenting with the initial ignition timing and advance, in particular changing the mech. curve on my Pertronix distributor with different springs. Cheers, good luck, and be careful not to drop anything down the carb. Ask me how I know!

I kept a running XL spreadsheet, tracking most of the changes I made..



 
#20 ·
I have the tools and the ability I'm just hesitant because I've never been good with carbs and never really been shown the proper way to adjust them and what to look/listen for. I know if you get the mixtures wrong you can screw up the motor.
 
#28 ·
you have to get the mixture and timing waaaay wrong to really hurt anything.. a few turns of a screwdriver and small twists of the distributor won't hurt anything and will show you what happens when you go off in the wrong direction..
 
#21 ·
As Derrick said, the Edelbrock book is very good. Just go one step change at a time. Do it in the order the books says, don't jump around. Make sure the engine is fully warmed up and choke is open. And pay close attention to changes. A rod change is typically 4% more or less fuel on either cruise or power (or both). That's not a whole lot, so pay close attention to whether it got better or worse and where in your test drives. Get the basic drive-ability dialed in before you worry about slamming it to WOT.

AND, get your timing checked and dialed in first. Several people have given you advice to start with timing. Do that first. Post back specs.
 
#23 ·
P40 good suggestion but since it runs awsome if I blip the throttle before WOT wouldn't that suggest the fuel pressure is good?
 
#25 ·
I'm going to buy the kit this week and try to play with this over the weekend if I can find time.

I was told they checked initial and curves (and more power could be found with a different advance spring )
 
#26 ·
Dustin, can we PLEASE get what the timing is they set it at?

We are sitting here guessing that you are under fueling and trying to get you more. Then, you suggest more power can be hard with an different spring and maybe you are over fueling. Doubt it, but you know...we are work with what we have.
 
#31 ·
Dustin, can we PLEASE get what the timing is they set it at?

...Then, you suggest more power can be hard with an different spring .
Another thing to consider, just what specs is this mechanic setting the ignition to? The "ideal" initial timing and curves will vary with the actual engine. I wonder, is the shop/mechanic comparing the curves to a stock 396 specs? and is your engine completely stock (aftermarket cam? intake? headers?, etc)?

Be aware that the stock ignition specs were set for emissions and might not be the best for performance anyway.
 
#27 ·
As I've mentioned in a previous post I will try to check it this weekend the person that did it does not have it recorded
 
#33 ·
From reading your initial post I get the impression that the problem may be on the secondary ignition circuit, in other words, the high voltage side. Perhaps your spark is breaking down somewhere. I'd look at your rotor, cap, points (if your distributor has them), wires, and plugs. As they say, 90% of all carb problems are in the distributor. :) I'd go over that stuff quickly and then start tackling the carb if the problem remains. The timing curve being off a little isn't going to cause a large stumble.
 
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