Needing some help with a friend's 56 Vicky. It backfires through the carb on acceleration like it's running lean, but the plugs are dry black indicating rich. I rebuilt the carb which is apparently from a 57. I also installed a Pertronix and found the dizzy is also a 57 type as the Pertronix is different for a 56 dizzy. The car ran good before Winter and when he took it out of storage it ran like crap. While working on it, it ran out of gas so the gas in it is new. To be safe I installed a new filter. I also installed a new distributer cap, rotor and plugs. I'll install wires tomorrow as these are 17 years old and feel a little brittle, not cracked, but getting there. I really don't think it's ignition and definitely seems like the carb, unless maybe a timing chain? I've never had a timing chain go bad. I've built a bunch of engines and always put in new Timing chains or gears so don't know what symptoms a bad one has. 
Here's what I have so far:
Engine: 292. He bought the car 17 years ago and knows nothing about whether it was ever rebuilt. He just cruises and is not hard on it.
Trans: FOM
Carb is Carter 4bbl AFB
Fuel pump puts out 3.5 psi while cranking it over.
Engine vacuum was around 8" but after new intake gaskets its now about 15. (Still low)
Vacuum advance works easily with a Mity Vac.
Compression test ranged from 85 to 115 with most around 100-105. This isn't real good I know.
Initial timing is 7* advanced at idle w/ vac advance disconnected and plugged at carb.
Starts easily and once you get past the carb coughing it revs well. Driving it is another story. Coughs a lot getting to speed.
Mixture screws were having zero effect and could be screwed in all the way (gently) and the engine still runs just the same. To me this would indicate running rich or flooding over. I've been working to get the idle as low as possible so as not to be running it in the transition stage. After adding the Pertronix and replacing the intake gaskets I can get some change using the mixture screws, but not what it should be at all. If I screw them in all the way it will kill the engine now, but it dies slowly not like you'd normally get screwing them in where it usually kills it fast.
I've rebuilt and been through the carb at least 3 times. I've sprayed carb cleaner through every hole, jet or opening and watched for it coming out the other end. Then followed that with compressed air. I've rebuilt a lot of older Holleys and do have a clue about carbs and this AFB, though different, really isn't a hard rebuild. To be safe I followed step by step using a Carter book by Dave Emanuel. I've also used other info like the Ford manual and whatever else I could find. I set the floats at 5/32" like the Ford manual and carb kit instructions say, but everywhere else people say set them at 7/16". That may be generic for Edelbrock carbs as they are almost the same. That is a lot of difference though for a float setting. I figured the Ford setting should be right for this particular carb.
I'm sorry this is so long, but I figured if someone was to take the time to consider what's going on, the more info the better.
Thanks in advance! Roy

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

Thanks for all the detail you’ve given.  It eliminates asking a lot of additional questions.

You’ve been into the carburetor three times so I would almost rule that out but this does sound like a carb issue.  The carburetor circuits could stand a good cleaning and especially if stored with ethanol laden gasoline.  I suspect a stopped up air bleed or two in the carb being the root of the problem.  A bad gasket under the fuel discharge nozzles will also upset the air bleed circuitry.  Also check for vacuum leaks and especially under the carb.  Double check that the intake manifold is seated properly and tight against the gaskets.  You might try doing a ‘running flush’ on the carb before removing it as that might help.  Here’s the link to performing that particular operation.

http://forums.y-blocksforever.com/FindPost51748.aspx

While likely not the problem you are looking for, the spark plug wires could stand replacing based on age and especially if they are carbon core.  Don’t forget the coil wire as it takes a beating compared to the spark plug wires.  Any dampness or oil on the wires can also be a problem.  Check that the spark from the coil wire is a nice crisp blue and not yellow or orange.  If not crisp and blue, then double check the ignition.  Poor grounding can also cause the symptoms you mention.

Lorena, Texas (South of Waco)

Thank you Ted. I will try the "running flush". I was doing some reading about the small passages in these carbs so I did go through it carefully with a wire to be sure. I'll check for vacuum leaks again now that it's been heated up a few times. I did notice today when I started it cold it ran great until the choke opened. Of course this make the case for it running lean and back firing through the carb. But, at the same time I don't have any adjustment on the idle mixture screws, indicating it's flooding. The plugs also look like the mixture is very rich. 

I don't know these carbs and keep thinking there's a common thing I don't know about. All carbs work off vacuum, but these Carters are different in that respect than the Holley's I've worked with. Adding to all this the low engine vacuum and poor compression test, it's likely a burned valve or timing chain too. Maybe several things together. The plug wires could definitely end up being the culprit.

Thanks for your help! Its greatly appreciated!
Roy

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

Roy, it's been 50 years since I worked at a Buick dealership as a tune up guy, but I remember working on AFBs.  There are driven-in aluminium plugs in the bottom of the fuel bowl or a fuel passage that, when they leak, leak raw fuel directly into the intake.  The hint that that was the case was idle adjustment screw ineffectiveness, rich running, poor engine vacuum reading, and poor fuel economy.  At least most Buick owners then didn't care about how much 30 cent gas they used.  The leaking soft plugs didn't cause backfiring on acceleration, that sounds like accelerator pump problems.  The plugs were installed at the factory to seal holes that gave them access to passages that they needed to drill.  If my old memory still works a little, it seems the plugs are below the main jets.  Also, are the fuel metering needles free to move up when vacuum drops on acceleration?  If they are stuck down, it would cause it to be lean on acceleration.  They don't have very strong springs under their pistons to raise them. I hope I haven't just added to your confusing situation.

John - "The Hoosier Hurricane"

Hi John. I will look at those plugs this weekend when I get into her again. Are they something I could tap with a hammer and punch in the middle to reseat them?
I partially opened the little covers over the rods while the engine was running and could see them moving fairly easily, except one was much less likely to rise than the other. The right side (passenger side) moved up and down a lot more than the left side. In fact it would go up and down rapidly where the one on the left would stay down. If I revved it, the one on the left would jump up at the same time it backfired through the carb so I figured the backfire was pushing it up. So, the left one could be sticking, at least a little, now that I'm thinking it through. I did try stronger and weaker springs but saw no real difference sitting in the garage. Could a rod go down too far and get stuck? I bought a rod, jet and spring kit, but none of the numbers work so I'd just be throwing parts in blind and hoping for good results. I suppose I could measure them and go for something that's close to start.  
Your help hasn't confused a thing and is giving me things that make sense to look at. I very much appreciate both yours and Tim's help on this! I'm better with Flatheads but I have a Y Block I built waiting to go into my 57 Ranch Wagon. It's got a 292 block with a modified 312 crank, .030 over bore, lumpy cam and dual quads (Edelbrock) so all this work now will help when I put that engine in. The dual quads are probably too much, but I have it and the set up looks great on the engine. I have a 3/2 set up as well if the 2/4 doesn't work out. A single 4bbl would probably be the best, but, we'll see.  I love the 57 you have for your in your message! Hopefully my little wagon will look as good!
Roy

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

If the plugs John mentions leak without a vacuum, you can clean the carb and then prop it up, either on a carb stand or make something to do the same, fill the float bowls and look for any drips.


Lawrenceville, GA

Great advice! That's something I should have done anyway to check for leaks, but didn't think of it.
This forum is awesome the way you guys jump in and help!
Thanks Charlie!

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

Roy, the plugs I remember were made of aluminium in various diameters in a drawer at the dealer.  They were short, around 1/4", and had a small head formed on them.  You tapped them in and the taper both sealed and secured them.  You could probably just tap the existing plugs to make sure they were tight unless the head is already against the casting.  We always tapped them when overhauling the carb just to be sure.  That sticky piston for the rod needs to be fixed, whether or not that is your actual problem.  I also got to thinking about your "dry black" plugs.  I've been working on my farm tractor, and the plugs always run black and the carb mixture is adjustable by turning a screw, so it isn't rich.  I blame the black on the alcohol in the gas, and in our area I recently found out that we have been getting E15 gas through our winter months, which means until June I'm putting E15 in the tractor instead of E10, which already blackened the plugs.  Alcohol burns with a black soot.  What I'm trying to say is that your black plugs may not be because of a rich mixture, which blows my leaking plug theory into the water.

John - "The Hoosier Hurricane"

I remember seeing small like freeze plugs under the carb, probably 1/4"ish, so I will check them out. On the black plugs, these are like heavily coated with black dust, like soot. I put in new plugs and they looked the same almost immediately. No more than 1/2 hour garage running with plenty of revs. The car was also smoking and smelled rich when he brought it to me and that seems better now, even though it's not right. In the past I've tuned over problems so don't want to do that and just cover up a problem. The owner is using this in a family wedding which has been a family tradition for all of his kids so I want to get it right as soon as I can.

I will say one thing, I wish the plugs were above the exhaust manifolds and the dizzy was up front like a late model flathead!

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

It would be nice to get the AFB sorted out but if it was Me and I wanted to get it reliable and running right I would replace it with a new Holley 4160 390 or 600 CFM . I don't mean a rebuilt but a brand new Carb that hasn't been messed with previously. New Ones I think are a very reasonable Price if You shop around. I bought a New 8007-0 390 on Amazon last Year. Not a lot more than a rebuilt. You already have the late Distributor and either a 57 "B" Manifold or Adapter on an "A" Manifold for the AFB Carb. I have done 3 of these and  the results are well worth the Cost. I understand that the challenge to try and get the existing Carb working but Sometimes the Alternative makes more Sense. It sounds like the trouble is Carburetor related. One Suggestion that I have done on mine is set initial Timing at 14 BTDC and limit Vacuum Advance to about 6-8. Centrifugal is 26 so total is 46 available. Starts easy and runs well. Low Compression they can stand more initial timing than something with higher Compression. If You do change Carb and up the Timing I think U would be impressed with how well it runs. Just my 2 Cents worth.