[taved]

Hi Folks,

I just installed a newly assembled 383 sbc into my 1972 Chevelle last weekend with advice that I obtained here in the forums :hurray:.

The engine is a 383, has HEI ignition, GM hotcam (hyd-roller), GM Bowtie Vortec heads, Edelbrock 800 AVS carb and otherwise a straightforward build. Static compression is calculated at 9.6:1. Compression tests give ~185 psi per cylinder. The best gas I have here in CA is 91 so that's what's in the tank.

I conservatively set the initial timing to 10 degrees BTDC (no vacuum advance). Once set, I hooked the vacuum advance can to manifold vacuum (about 15 psi).

The engine starts and idles fine (set high at 1000 rpm), but nearly stalls when put into gear. The car drives around fine in idle, but as soon as I crack open the throttle, the car falls flat, nearly dies and sometimes will backfire out the pipes. I had this engine on a run stand after the build, but this is the first time I've had it under load.

If I can get the car up to speed, I can open it to WOT and the engine runs well. Inspection of the spark plugs shows good timing by the discoloration of the ground straps right at the bend of the straps.

I have a wide-band air/fuel gauge on the engine and it goes lean when the car nearly dies. This is why I was thinking that this is a carb-related issue. The accelerator pump kicks in right away when the throttle is opened (nice, strong squirt action- new carb) and I even lowered the plunger arm/rod hole- no effect.

Now, I'm wondering if this is a timing issue. Is it possible that my initial timing is not enough, the engine runs well at idle with the vacuum advance, but as soon as I crack the throttle, the vacuum drops, the timing retards and then engine falls flat? Should I try ported vacuum or just give it more initial?

Thanks in advance.
Dave

 

[Chevyobsessed]

"Falling on it's face,..backfiring through the exhaust,..runs ok at WOT,..A/F ratio goes lean when it nearly dies",..etc

You have a vacuum leak.

 

[Tom Mobley]

forget the religious wars, just move the hose to ported vac and see what it does. if it is fixed leave it there.

Holley 670SA will fix it, wanna bet? Check the Eddie carb for accelerator pump functioning.

 

[novaderrik]

ignore the air/fuel gauge and give the engine what it wants.. after you get it running good, you can then use the gauge to see where it wants to be under different operating conditions.

the first thing you want to do is read the book that came with the carb.. if you didn't buy it new and don't have the book, call Edelbrock and have them send it to you... it will explain to you what each system in the carb and ignition does and how to tweak it all to work perfectly together on your engine..

is the electric choke hooked up and working properly?
baseline everything- idle mixture screws 1.5 turns out from gently bottoming out, idle speed screw set so the idle transition slot is square, and put the initial timing to 16btdc or so. leave the vacuum advance on full manifold vacuum...
make sure there aren't any vacuum leaks- it's easy to forget to put a vacuum cap on an open port. also put a wrench on all the intake manifold and carb mounting bolts..
fire it up and start tuning. part of the fun of these things is the tuning and tweaking- it might run like crap and no matter what you change it doesn't fix it... then suddenly you will turn just the right screw to just the right place and it will just magically get better and it will idle silky smooth at 600rpm in gear, have instantaneous throttle response, pull hard from off idle to 7000 rpm, and get 24mpg when you drive it nice..

from my experience with the HOT cam in a 10:1 motor with vortec heads backed with a 4 speed and 3.70:1 gears: it liked 16BTDC initial, 36 total, and as much full manifold vacuum advance as i wanted to throw at it..

 

[Verle]

Do what Derrick said as a starting point and do fine tuning from there.

 

[658Chevy]

Make sure the choke is adjusted properly. If the choke if not set up right, it can cause the bog after warm up. It happened on my Edelbrock AVS, with the exact symptoms you're experiencing. Make sure the choke arm to the choke blade is allowing the blade to stand completely vertically, with no forward tilt. If the choke blade looks like it's not completely vertical (look closely) and you can push the blade back against spring tension an eighth or a sixteenth of an inch, that's your problem. The choke blade must be completely vertical and have zero give when pushed back with your finger. First, adjust the choke spring tension per the Edelbrock manual (it's done while the engine is completely warm and running; a little different procedure than other carbs I've had), and if that doesn't do it, the choke arm will have to be adjusted by bending.

I hope that's the problem as it's a simple fix. Also check the secondary opening. On the AVS carb, that's also a simple adjustment.

 

[taved]

Ok folks,

I moved the vacuum advance over to ported vacuum- not much change. BUT my initial advance was only ~5 degrees. I upped this to 15 and the engine now runs up pretty well. Moving back to manifold vacuum did not make much difference- at least not in noticable drivability. I will try this back and forth again at some point.

There is still a lean stumble, but the car does not fall flat. I could not track down any vacuum leaks. I'll look into the choke adjustment- I had not seen this suggestion until I came back in.

I also received in the mail today a set of accellerator pump nozzles, but I'll try optimizing the choke and timing settings before going there.

I did not rev past 4K as I have my rev limiter (MSD HEI module) set there until I'm sure that this distributor does not show timing advance creep past 35 degrees.

Thanks and I'll report back again.

Dave

 

[Geo71]

If the AVS carbs are like the Performer models, make sure your fuel pressure isn't too high. I think it is 5.5 or 6psi max.