Team Chevelle banner

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

6446 Views 7 Replies 7 Participants Last post by  Geo71
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.
See less See more
1 - 2 of 8 Posts
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.

See less See more
1 - 2 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.