Off-idle engine falls flat/backfire due to timing?

6444 Views 7 Replies 7 Participants Last post by Geo71, Jul 12, 2013

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Discussion Starter · Jul 11, 2013

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

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658Chevy

· #6 · Jul 12, 2013

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.

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