383 not running as good as expected

[PtitRaisin]

Hello all from south Florida,

I think I spent enough money and time fucking around with my engine, and now it's not running as good as expected.
And it's time to ask for help.

So, here is the story:
I have a 383 with a 727 and a 8-3/4 rear end.

I recently rebuilt the engine completely

- Re-bored +0.020"
- Ground crankshaft
- New rods
- New pistons
- Stock exhaust headers (the bigger ones though)

Edelbrock #60825 heads:
Combustion Chamber Volume (cc): 75
Intake Runner Volume (cc): 210cc
Exhaust Runner Volume (cc): 70cc

Edelbrock #7194 cam and lifter kit - also changed the valve springs to single #5792 as recommended by Edelbrock to preserve my camshaft
Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 1,500-6,500
Intake Duration at 050 inch Lift: 238
Exhaust Duration at 050 inch Lift: 246
Duration at 050 inch Lift: 238 int./246 exh.
Advertised Intake Duration: 300
Advertised Exhaust Duration: 308
Advertised Duration: 300 int./308 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.480 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.495 in.
Valve Lift with Factory Rocker Arm Ratio: 0.480 int./0.495 exh.
Lobe Separation (degrees): 110

Harland Sharp S70015K
1.5 ratio

Crane cams pushrods
8.180 long (measured with pushrod length checker before ordering)

Weiand dual plane intake 8008WND

MSD distibutor 8386
MSD 6AL ignition control

My first choice for carb was a 750CFM Edelbrock.
The guy who machined my engine told me to set the timing to 20deg BDC and check if the centrifugal brings it to 35 at 3500RPM.

It was not running perfect but it was okay. Rough idle, Was almost impossible to get the AFR right... but I blamed it on the camshaft.

So I put 1200 miles on the engine with this configuration.
I didn't have to take her to a dyno to realize that she doesn't have the ass-kick I was expecting.

A friend of mine told me since he installed a Holley double pumper on his car, it was way better. Also 750CFM were maybe a little too much for my configuration.

So I went for a Holley carb:
Their calculator suggested me the Street Avenger 670CFM. So I went for this one.

But now... It's even worse.
Carb sputtering, back fire in the intake at low RPM.

I played with the timing again, put it back around 15 deg. Does not change much.
I have welded bungs for O2 sensors, on my drop tubes, I have a AFR meter that gives me each side:
Arount 14/15 at idle (rough idle so it's fluctuating) and its dropping 13.5 when I accelerate slowly.
I have 8" of vacuum @1000RPM (Edelbrock cam specs say 10"@1000 normally)

Questions:
Do you think this camshaft was a dumb idea? Since I didn't change the headers.

Does it sound like a vacuum leak somewhere? (Could not spot anything with carb spray)

Does it sound like a carb adjustment problem only?
Is 670CFM still too much?

Is it a timing problem?
I didn't play with the MSD box now. Although I think you can only adjust the rev limiter.

Am I missing something I forgot to check?

I hope that's not too much to digest.
Here is where I am now, I'm kinda stuck.
All input welcome, maybe I'm just stupid!

[Kern Dog]

First up, I have never heard of anyone that makes new pistons only .010 oversize. The smallest overbore that I know of is .020.
The reason that I mention that is that you may have a really low compression ratio. If that is true, then camshaft choice was way too big.
Many replacement pistons for 383s have a terrible compression height. The generic pistons are actually shorter than stock pistons, making even less compression.
Check this out: A 1970 383 2 barrel was rated at 8.7 to 1. I just tore down a stock 1970 383 2 barrel, the original engine from my '70 Charger project. Calculating the numbers with everything right in front of me, the numbers actually came to 8.12 to 1. Yeah....Over half a point under the published number.
Then, if I were to just reuse the stock pistons that sit .076 below deck with the popular Fel Pro .039 head gasket, the compression ratio drops to 7.85 to 1 because the Fel Pro is almost double the thickness of the stock .020 steel head gasket.
I'm not trying to baffle you with math. It can get easy to get lost in the numbers and not learn anything.
In short, you need to know which pistons were used, what the deck clearance was, what your combustion chamber volume is, head gasket thickness and with all of that, you can calculate your compression ratio. I suspect that you are no higher than 8.5 to 1. If this is true, a smaller cam would help a LOT.
No street car with an automatic needs a double pumper carburetor either. They dump too much fuel into the engine and wash the oil from the rings, causing premature wear and bad gas mileage. A vacuum secondary Holley or Edelbrock 750 is a decent carburetor. A Thermoquad is too. Remember, the 318 4 barrel Police cars came with 800 cfm Thermoquads. A 383 needs to rev a bit to make power so the 750 vacuum secondary will work if tuned right. The vacuum secondary carburetors only give what the engine asks for.

[c00nhunterjoe]

As stated, what part number pistons were used, part number head gasket or at least the thickness. Odds are its low low low compression with a big overlap cam that makes your dynamic compression potentially into the high 5s or low 6s. What gears are in the rear and what converter? If you have a low compression 383 with a big cam, stock converter and 3.23s, it will feel like a dog until it gets into high rpm. Any carb will be difficult to tune due to the low plenum response. Lots of people run double pumpers on automatics. The popping you are getting with the holley is most likely a lean pop- tuning. I have yet to see an out of the box holley that didnt have a lean bog. I personally dont care for the street avengers, but the principles are the same. The eddy carbs are good too. Just have to tune them. And i dont mean turn the screws on the outside with either carb.

[XH29N0G]

 

I put a 670 street avenger on a 383 with the stock cam and had backfire through the intake (lean).  I needed to increase jet size and tune the accelerator pump to fix. It will be easiest if you can log your AF.  Don't make the same mistake I did (which was to look at every single number and blip as if it needed to be perfect).  I should have used it for general information about the AF at idle, the AF at cruise, The AF at WOT, and the AF for the transition from cruise to WOT.  I am sure the avenger can be made to work.  I believe a small double pumper can be made to work, but I think the vacuum secondary is probably better in many cases.

When I was looking I heard there were better cams for the 383, but you want to know the CR before deciding. I used one with shorter duration and it worked well. My 383 was never crazy powerful, but was good for me. I know some make them run very well. I think the 75cc heads will work in your favor.

[c00nhunterjoe]

If it turns out to be low compression with a 3.23 rear and tight converter, the double pumper will be hard to tune and a vaccum secondary will work light years better in that situation. The 75cc heads will help with the compression, but being aluminum will negate 1 full point of compression that the engine "sees" so if it measures at 8:1, with aluminum heads it becomes 7:1. Take a large overlap cam and dynamic drops even further, thus the issues at hand.

We really need to know the pn piston and thickness of the head gasket to get a rough idea of where you are at. If you cannot get that, do a compression test on 2 cylinders, report the numbers and where you live so we know the elevation. From that, we know your cam specs and can relatively accurately calculate your compression ratio.

[PtitRaisin]

Thanks a lot for your replies.
I really want to fix that, and above all understand what I'm doing.

@Kern Dog, you are absolutely right: 0.020. I corrected my mistake in the original post.

Let me go through my papers to get pistons height and cylinder head gasket thickness.

@c00nhunterjoe you guessed right: stock converter and 3.23 gear, and it's exactly how she reacts --> "it will feel like a dog until it gets into high rpm"

Now I'm looking for the "not too much to dismantle" solution.
But if it's necessary, I'll take the engine out and apart again.
Since I was planning to switch for a manual transmission, I'm open to any upgrade that will restore its badge of "muscle car".

[c00nhunterjoe]

Lets wait to hear what pistons are in it, and then go from there as that one factor will be key in any direction you take from this point forward.