stock 1968 440, rough idle

[DAmatt]

Hi folks, finally the car is home after a 5 1/2 year complete restoration, so you can immagine my enthusiasm!

Unfortunately though, the car shakes pretty bad, esp. at idle. Engine was rebuilt to stock specks, i.e. L2355F pistons & stock Mopar Performance cam. Only thing I did was block the heat crossover through the pan gasket, and disable the choke on the original Carter AVS which was professionally rebuilt by Scott @ harmsauto.

Now I understand that the car is a bit cold blooded if you take the choke out,  but still it idles rough even after warmed up.

Ignition is about 14 degrees initial, and 32 or 34 at 3000 (very hard for me to get a good read).

I took the LH thread idle mixture screw all the way in, and back up 1 1/2 turns. Hooked up the vacuum gauge, it shows aroung 16 in Hg vacuum. I uploaded a short vid on youtube, please take a look and lend me your thoughts. Is this what it's supposed to sound and vibrate?

https://youtu.be/Pn48CSw4KHs

I must be doing something wrong, please help.

Thanks!

[XH29N0G]

Wait to see what people say. 

When the 383 was stock in my car, it was always a little uneven compared to a 2bbl 350 Pontiac I had.

I am having difficulty from the video telling if it is a uneven or not. 

You describe how you set the idle mixture screws.  Have you optimized them for the highest vacuum at a specific RPM?  Have you tried the same with the timing and seen if it is much different from your setting of 14 degrees.  I would think 14 would be good, but just asking?

The other thing I would check and this is only because I cannot detect a miss is to see if the idle gets worse when each plug wire is disconnected.

Again - wait to see what others say.  I am not a mechanic, just sharing thoughts that come to mind.

[70 sublime]

How old is the gas you are using ?

Did not sound too bad in the video

Can you take the car out and drive it to see if it is more happy after a good run ?

My 383 ran crappy after sitting for some time but got better the more I drove it

[chargerbr549]

The LH single screw is and adjustable "air" bleed and sometimes will only do so much, there should be two RH idle mixture screws down below on the base plate that adjust the fuel and they should help you dial your idle in. They might be hidden under some small plugs? 

[maxwellwedge]

If Scott at Harms did the carb - it should be very close to being adjusted properly - as he runs them.
Check the usual stuff first - plugs etc.

[Just 6T9 CHGR]

What are the specs of this "stock" Mopar Performance cam?

My 440 runs like that (if not worse) due to the overlap in the cam   I purposely had it ground to produce a lope at idle

[DAmatt]

First of all thank you all for your replies!

Now on to a short disclaimer, this is the first American car I have touched, first carb, first points ignition. I am also located in Transylvania, where nobody knows how to work on these cars. Up until 3 days ago, it seems I didn't even know how to read the timing, the timing gun had an advance correction wheel, which was set at 10 degrees, so the marking on the block showed 10 degrees LESS than actual. When I took it to zero, I was shocked to see the initial advance at 24-25 degrees, and climbing well above 40 when revving a bit (vacuum advance disconnected & plugged). Perhaps that had something to do with flames exploding through the carb...

I understand my current level of competence, and am approaching this with a humble attitude, while being very open to learn, and to any advice lent. I learned how to adjust the initial timing by rotating the distribuitor, now I can confirm that idle is @ 14 degrees, and revving it, it shows 32-34. Vacuum remains at around 17 no matter what I do to the idle screw, all the way in or 4 turns out. I'll look for plugs in the area that the 2 screws are on the Edelbrock, but I have to admit I am weary of taking apart things I'm not confident with...

I think once the engine is warm the carb backfires are gone (still need to confirm), it's still a problem when cold, but I think it's because of the choke delete. Will post back on the matter. However, a problem that wasn't cured was that if you stab it from idle, the engine bogs down, and almost dies. Now considering Scott's reputation, I am ruling out the carb, but perhaps there are adjustments that can or should be made? Or are all these symptoms of a new engine, still needing to be broken in?

An interesting fact is the considerably different sound the car has with the Edelbrock vs the Carter: 6T9 CHGR, you mentioned a lopey idle, that's exactly how it sounded with the old, non original 2.25 exhaust & the Edelbrock, if you look on my channel for a 42 second clip. But in another one from 8 months back, same old exhaust & Carter carb, sounds totally different. Now the car has a complete Accurate Exhaust system fitted.  From what I can see from the sheet the builder gave me, cam is MP 4452783 with 450 intake, 464 exhaust, 214/255 duration @ .50, and a 113 degree centerline. I wanted a stock rebuild, to feel exactly what they felt when they left the lights back in '68. I had a P100D Tesla till recently, so an authentic feeling was more important than outright performance, so stock is definitely what I pursue. I just want to get it working, to be able to enjoy it the way it's meant to be driven!

Unfortunately, problems arise, now sometimes the starter doesn't crank, and I've got a power steering leak somewhere (need to know what fluid to use). Hopefully I will learn to properly maintain this car, and be able to keep it in tip top shape, at least for the next 50 or 60 years!

[DAmatt]

UPDATE

I made a short clip after the engine has fully warmed up.

What do you guys think might be wrong?

https://youtu.be/p0uGJXS4ONs

[XH29N0G]

I find it difficult to diagnose by video, because the sounds are a little distorted from inside the passenger compartment, but I hear the backfire.  Wait for someone else to reply before following my suggestion because I could be wrong.

I am assuming since the engine was just rebuilt that your ignition is fine.  This is a big assumption because if it is off, then you will be working with the carburetor and not addressing the real problem.  If it is the carburetor, a lean backfire is typically through the carburetor when the throttle is depressed quickly.  There can also be an effect with timing, and you need to know that you are measuring it correctly, but I think you are.

I do not know your carburetor and am mostly familiar with Holley-type carburetors.  I think the basic principles are the same.  There are at least three things the carburetor needs to do. 
(1) Provide the right mixture at idle and cruise,
(2) provide the right mixture at wide open throttle, and
(3) provide the right mixture when the throttle is depressed. 
 
The first is controlled by the idle circuit and extends up to about 2500 RPM
The second is controlled by the main cirucuit (in my case Jets) and extends from about 3500 RPM and higher.
The third is controlled by the accelerator pump.

There are other things that factor in, but these are the three.  The spot between 2500 and 3500 RPM is a little complex

What I would do if I were you would be to:
(1) confirm the idle mixture is correct and you do that by vacuum.  It is a troubling that you see no change in vacuum with the idle mixture screws.  My engine will stall if I bottom out those screws and definitely changes if they are too open.  You want to make sure those are set OK, and they need to be open enough to give enough gas so the car doesn't stall and backfire.
(2) confirm that you have a good accelerator pump shot.  This is needed to make sure that enough gas is squirted into the engine when the throttle blades are open so it does not backfire and spit.  You should be able to see this with the engine off if you move the throttle by hand.  It needs to be instantaneous.
(3) the third thing is the high speed circuit.  If this is too lean, it can also cause a backfire.

See what other people say and read up on the web on this for your particular carburetor.  It is a bit like a puzzle, but I am sure it willb e figured out.

[DAmatt]

I also took a photo of the carb, circling two holes that seem to be made for screws... are there supposed to be any screws in there? Could that be the problem? I can once more confirm vacuum does not modify when fiddling with the top middle screw. That is manifold vacuum..
And yes, the back fire is happening mostly when I stab the throttle. Revving it slower makes it happen much less often.

[Just 6T9 CHGR]

That must be a different model/style AVS carb than I had on my 440 at one point.   I had the stock "correct" #4640SA carb (1969 440 w/AC) rebuilt and it had 2 idle mixture screws in the location you circled.   It did not have the singular screw in the center that yours had....
Not saying that its a problem, just answering the question you posted about the screw holes....

[XH29N0G]

As I mentioned in a previous post, I have not worked with Carter carburetors before so someone else will know the answer better than I do. 

Nevertheless, I did a quick search about the screws and see two conflicting explanations for the two v one v 3 screws.  One explanation is that there was a transition from two to one idle mixture screws sometime in 1969.  The other explanation is that the top one controls air and the bottom two fuel.  I do not know which is correct. 

Another thing that I saw was that these screws (or at least some of them) are left hand thread, which would change which way you turned them to close the channel by bottoming them out. 

Since this thread has migrated a little to adjusting the idle mixture, If you do not get a response to this thread in a day or two, I would start another one with a title about adjusting mixture screws on a Carter carburetor. 

[DAmatt]

From what I have read, the 1968 only Carters had the idle mixture screws set from the factory, and then caps put over them (that's why they have the single screw on top, which is either a mixture screw, or, as somebody over here hilariously said, an adjustable vacuum leak). While I initially thought there aren't any screws, actually I DO have the 2 screws (look at the pic!), only that they're inset, with about 10mm to the edge - someone took the caps off, to be able to adjust them. That same somebody must have also blocked the single LH screw, and that's why there's no change to the vacuum gauge.

Now I took the 2 small screws all the way in, than 2 turns out, and the carb runs better. However after starting, the car still stumbles when nailing it from idle. However, if I give it full throttle for a few seconds, after that the stumbling goes away. Is that because at first, secondaries have no fuel, but after that, fuel reaches the secondaries, and troubles are gone?

In any way, things are improving a bit, and after I get the car registered, I'll take it on longer drives, let the engine rev a bit higher, and report back.