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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 09-10-2006, 12:11 AM   #1
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Troubleshooting my Edelbrock 1406

Hey, guys. I need some opinions on where to go next with this. First, I'll tell you what the symptoms have been and what I've done trying to narrow down what the problem could be. This is gonna be long, sorry.

I have a carbed L03 with a fairly new 1406 Edelbrock, aftermarket dual-plane intake, Mallory coil-in-cap HEI distributor. I don't know a lot about what's been done to the engine. The car has 70,000 on the clock, a rebuilt 700R4 and a modded rearend. With everything else I keep finding that's been changed, it wouldn't surprise me at all if it has a weird cam, but I really have no idea. It's got an aftermarket oil pan and timing cover so I think the engine's been pulled and messed with at some point. It runs damn strong for a 305. My only problem is at idle.

When it's idling, particularly if it's in gear and the AC is on, I get a steady shake. The whole car shakes and I had a problem with it surging at low RPMs. Now, I thought the surging and shaking might be two seperate problems, so I approached them as such.

The surging made me think I probably had a vacuum leak. Lean at idle perhaps. So the first thing I did was the old propane test. Checked the intake bolts and tightened a couple of them, checked around the carb, the vacuum hoses. Replaced the carb gasket. I found nothing. I don't think there's a vacuum leak, at least not outside the carb.
I messed with the idle screws on the front of the carb, and that's where things got weird. I'll get back to that. But I've pretty much cleared up the surging problem by adjusting the idle screws.

Now the shaking. Engine mounts are fine, transmission mount is fine. It acts like it's missing on a couple of cylinders, but only at idle. First thing I do is dive into the ignition system. Changed the sparkplugs. I was expecting to find at least one of them fouled out or otherwise FUBARed, but no. A nice tan color on the electrodes, a little bit of carbon. Just look like old sparkplugs that needed to be changed, all of them were the same. I'm going to change the wires tomorrow just for the hell of it, but I don't expect that will change anything. I pulled each wire from the cap while it was running and they all arced. I haven't (yet) changed the cap, coil or module. They're all at least as new as the distributor. I've adjusted the timing, moved the vacuum advance hose, run without vac advance, but no matter how advanced or retarded the timing, there is no effect on the shaking at idle. I think it might not be an ignition problem. You'll see why in a minute. Let's get back to the carb.

I got an air/fuel ratio gauge. Narrowband, the type you attach to the O2 sensor. Now, I know these things are about as accurate for tuning as calling a psychic hotline, but I figured it would at least tell me whether I'm running lean or rich. It does. I'm slightly rich at WOT, probably because I changed the metering rods and springs, and I'm dead center of the gauge at cruise. At idle, it was lean as hell. I mean it barely registered on the gauge. For a while I thought that might be normal, but then I got to messing with those idle screws again. I got it to idle a good bit richer and now the surging is pretty much gone. The shaking is better, but it's not fixed.

Here's where it gets strange. I know the proper procedure for adjusting those idle screws, but that just doesn't fly with this carb. The passenger side screw has no effect on anything. If I turn down the driver's side screw to the point where it barely runs, then open up the passenger screw, nothing changes. Idle doesn't go up or down and nothing changes on the A/F gauge. So I just set the passenger screw about 2 turns open and focus on the other one. I start turning that thing out, it starts idling better, I keep turning until there's no change, and I check the gauge. Still very lean, still surges when I give it a little gas. Finally I cranked that screw about twice as far out as it should be, and I shut down the passenger side screw almost entirely. Now it idles with the gauge right in the middle and there is no surging. It still shakes at idle. More specifically, it shakes when under a slight load. As with the AC on and in gear. Runs a lot smoother at light throttle though.

Clearly there's something wrong with this carb if the passenger's side idle screw doesn't do anything. But I figured if I compensated for that by opening up the other one it would be okay. I was partly right, since that was the only way I could get my gauge to stop reading lean at idle, but then a thought occured to me: The O2 sensor is on the driver's side manifold. I'm not reading both sides. Maybe the driver's side cylinders are fine and the passenger's side is running lean. Could that be the cause of the shaking? Is that even possible? When the carb moves out of the idle circuit it becomes perfectly smooth. There could be other causes. Maybe it's over-cammed. It doesn't pull very much vacuum, but then it is a 305. Or maybe it is something in the ignition system, I don't know. What else should I check?

Either way, I think I'm going to have to take that carb apart. Something's not right with the idle circuit, or a float is out of adjustment, something. This is my first Edelbrock carb. I've used Holleys in the past, and I know I can at any time just get a used Holley, put a $30 Trick Kit into it and be done with it. But if I can fix this one I'd rather do that. Does this sound like a common problem with these carbs? I looked at the prices of rebuild kits for this thing, and I saw a $40 one that was basically a gasket set, and another kit for about $100. Which kit should I get? Do I even need one? If I get into the $100 range then I think I'd rather get into familiar territory and rebuild a Holley instead, but maybe this is a simple fix.

Edelbrock/AFB wizards, I await your wisdom. Thank you!
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Old 09-10-2006, 11:17 PM   #2
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Well I changed my plug wires today, and that was no help. I'll dig into the distributor soon, but probably not before I take care of the carb situation. Can anybody tell me what to check when I take the carb apart?
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Old 09-11-2006, 04:17 PM   #3
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I would focus on,
the idle mixture screw, not doing anything

are the off idle slots clean?
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Old 09-11-2006, 07:47 PM   #4
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Don't know, but I will find out. Where are these slots located?
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Old 09-12-2006, 04:39 PM   #5
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there are two small holes in the primary, above the butterflies.
a round hole just above the butterfly,
a slot above that.

the slots are onthe 'outer' side of the primary bores.

I am not saying this is the only problem,
but it needs to be fixed anyway.

fix the obvious problem first.
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Old 09-12-2006, 04:57 PM   #6
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i have a 1406 on a carbed 355. had similar troubles when i fisrt put it on but after tuning it its fine, you need check the floats reguardless of how the car runs. buy a tuning kit for the carb, read your manual on how to adjust things and go from there. with a little playing aroud you will have good runnung and idleing car. provided that the problem is the carb and not the engine itself...
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Old 09-14-2006, 09:03 PM   #7
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Engine's fine. I borrowed a Holley and the problem went away. At this point I could follow my gut and trust in brand loyalty, go the "just get a Holley and be done with it" route, but I've studied the diagrams, read everything I can, and I really appreciate the simplicity in the design of this carb. It has frustrated the hell out of me, but it obviously has a problem. I do have the tuning kit and I've adjusted everything that can be adjusted without disassembly, so now I'm just going to fully rebuild it.

I got kit #1477, and tomorrow night I'm going to tear it all down, RTFM and get this thing straight once and for all. I'll let y'all know how it goes.
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Old 09-14-2006, 09:31 PM   #8
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Car: 86 Trans Am, 92 Firebird
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Axle/Gears: Moser 9", 3:70 trutac, 3:23 torsion

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yea, ive torn into my carb so many times i can do it in my sleep, always trying to make it just that much better, it seems to work well but it took a lot of fiddleing to get it right, i mean finally i get the idle right, then have to work on the primary set-up then get that right, then work on the secondary's, get them right, it took a while.
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Old 09-14-2006, 09:54 PM   #9
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Well, if I can trust the "*** dyno" and my A/F meter, the carb is otherwise pretty well dialed in. The accelerator pump shot is perfect, there's no stumbling, the AF meter dead middle of the gauge except in the secondaries. At WOT it's slightly rich, but that's okay. I want it like that. It actually runs better than the Holley I tried, except at idle.

Of course, I'm sure I'm about to screw all that up when I fix whatever's wrong, and I'll have to adjust it all over again. No big deal though. I'm glad I have that gauge.

This is obviously the first time I've opened one of these up. Aside from just following the instructions and making sane guesses for the initial setup, can you offer any advice on the rebuild, or is it pretty simple? It doesn't look that difficult...
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Old 09-15-2006, 03:00 PM   #10
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Car: 86 Trans Am, 92 Firebird
Engine: 408 sbc, 3.1L of raw power
Transmission: TKO600, T5
Axle/Gears: Moser 9", 3:70 trutac, 3:23 torsion

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if the carb work good for you at all times except idle there really shouldnt be any need to rip into it... if the idle screws do nothing when you adjust them, thats not right, what fuel pressure do you have? do you have a regulator? do you have a stock pump? if you have the car idling and you stomp on the gas does the motor fumble or does it work right?
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Old 09-15-2006, 06:00 PM   #11
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As soon as I give it any throttle, it romps. I'm running about 5 pounds of fuel pressure through an adjustable regulator. Stock TBI pump.

As for the idle screws, the driver's side screw works as it should. The passenger's side screw does nothing. It was surging pretty bad between idle and ~1500 RPM until I opened the driver's side screw to about 5 turns.
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Old 09-15-2006, 06:07 PM   #12
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Car: 86 Trans Am, 92 Firebird
Engine: 408 sbc, 3.1L of raw power
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if the passenger side screw is not doing anything its probably the problem, starving one side of the engine at idle. check the manual see if it show a diagram of the mixture screws and how to clean them.
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Old 09-16-2006, 03:27 PM   #13
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Well, I rebuilt it last night. Tore it all apart and spent about six hours cleaning every crevice, hole and passage. Even stuff that's not functional. It was pretty gummed in some places. The primary venturi on the passenger's side had some clogs and the passenger side float was hanging way too low. The bowl on that side was rather dirty. I just got in a zone and slaved at it. It looks like a new carb now. I lubed everything that moves, got it all into spec, used the rod and jet combination that I thought it would need, and then I went to bed.

I put the carb back on today and I was amazed. It's never run so well. Awesome throttle response throughout the RPM range. The ratio seems to be dead on. I'm quite happy with it, should have done this weeks ago.
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Old 09-16-2006, 04:59 PM   #14
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Car: 86 Trans Am, 92 Firebird
Engine: 408 sbc, 3.1L of raw power
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congrats
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Old 09-16-2006, 04:59 PM
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