1k RPM gentle acceleration backfire and stall (animation!)

[Dood]

Problem in a nutshell: at just under 1k RPMs under weak acceleration, the engine misses and backfires through the carb and often stalls.

Thanks for anyone who suffers through this post. I put in some pictures and an animation to make things more interesting.

The configuration: 318 block, chrysler 360 heads, chrysler 360 intake, holley 4360 (450 CFM) carb (just rebuilt), mechanical distributor (just installed rmfd), MSD 6A ignition box, MSD blaster 2 coil. Let me just point out that the long block and ignition setup were in the truck when it came to me. Don't blame me for them! I don't know what if anything has been done to the block to accomodate the heads. The casting numbers can be found here, on my build thread. I don't know the cam. I did rebuild the carb and put on a new (rmfd) distributor.

More details with images and animation:

When the engine is cold and I start it without the fast idle cam dropped on the carb, it will fire up w/ about 13 to 15 inHg on the vacuum @ about 500 RPM and run for a second or two during which the speed and vacuum falter and drop steadily until it stalls. Do this twice more and it will stay running and idle roughly at about 650 RPM w/ 15 inHg on vac gauge. If I put it in neutral and let the clutch out, it will drop to 500, the vacuum decreases to < 14 and often it will stall. For giggles, I looked at the timing during this stage and it was > 5 ATDC (that's right, after).

Cold start tach, vacuum and timing

coldstart_small.png (73.6 KiB) Viewed 587 times

If I try to accelerate gradually, at just under 1k RPM, it will miss and backfire through the carb and often stall. (Now my freshly cleaned and rebuilt carb is black inside, my air filter is dirty and the clear vacuum hose I put on the vacuum gauge is black inside.)

1K RPM stall pathology animation

stall.gif (177.59 KiB) Viewed 587 times

Fast forward 5 minutes of idling in the driveway and the idle speed will be closer to 800 RPM and the timing closer to TDC with vacuum ~ 15 inHg. The engine is warm but I haven't driven it and the RPMs haven't exceeded the idle. Now, if I put it in gear (or leave it in neutral) and try to accelerate gently, at precisely the same RPM (~1k) it will backfire and almost certainly stall. If I got it rolling, I can pop the clutch to get it started again. Often, I can double pump the accelerator to get it past the 1k kill zone and into 1500 to 2000 RPMs, then burn the clutch to get it moving. Not my favorite way to start out the day.

Warm, undriven w/ RPMs not above idle

warm_undriven_small.png (75.48 KiB) Viewed 587 times

Now, fast forward to the first stop light. I've had it up to 2000 or 2500 RPM on the way and at the stop light, the idle has risen to just over 1k RPMs. If I check the timing now (yes, I've done it at a stop light in the predawn hours), it will be ~ 8 BTDC and the vacuum can be 18 inHg to 20 inHg. As the idle is above the dead-zone in RPMs, it will accelerate fine with no backfires.

Warm, first stop light at idle after hitting 2000 RPMs

warm_driven_small.png (78.45 KiB) Viewed 587 times

If I leave the clutch engaged as I'm pulling up to a stop light and bring the total RPMs down below 1k RPMs before I put the engine in neutral, it will idle at a lower speed, i.e. 800 RPM (15 to 17 inHg) and I'll have to suffer through the 1k stall zone. Deceleration does not trigger the stall as It drops below 1k on accel. Same thing happens if I change from 1st to 2nd early and the RPMs in second fall below 1k RPMS.

Warm engine break to stop tach, vacuum and timing

warm_enginebreaktostop_small.png (78.05 KiB) Viewed 587 times

I've found and fixed a couple of vacuum leaks, e.g. the carb choke pulloff can (converted choke to manual), the distributor vacuum advance can (put on a new distributor and can, thanks to robertob for pointing me at the can). After it is warm, regardless of where the idle is, at idle the vacuum is steady. I rebuilt the carb. New plugs, filter. Strong spark. All of this has improved power, smoothness, etc. above 1k. The dead band remains unaffected.

OK, so the question, so what is the origin of the 1k accel dead zone? I'm also curious why my idle is dependent on the previous state, e.g. engine speed fast (i.e. > 1k) or slow (i.e. < 800 RPM).

So, one more thing, possibly a red herring: When the engine is cold and I'm in that low RPM idle state I hear a loud 'clack' from the driver side of the engine, at (I think) engine speed. As I mentioned, for giggles one morning, while it was still cold and making this noise, I put on the timing light and it was > 5 ATDC, maybe as much as 10? I quick-like loosened the distributor and advanced the timing 10 to 15 degrees and the noise all but went away. I put the timing back to where I had it from when I set it while it was warm. If the engine is warm but in that lower idle speed state, this sound will be heard, though more quietly.

[wally426ci]

Holy crap, you just blew my mind. Sorry I cant help!

[oldandintheway]

Timing chain?

From what I see, your timing is all over the place. Shouldn't you be at about 10 btdc at idle, and then it should advance (further btdc) from there under acceleration ?

It's been years since I had a similar problem, but here is what I did;

manually rotate the crank (using the balancer bolt) until you reach tdc , with your dist cap off and noting the position of the rotor, then rotate the balancer in the opposite (btdc) direction until the rotor just starts to move. Note the timing mark indication. If it is more than a couple of degrees (you might be able to get a spec on allowable 'slop' from a good mechanic or shop), I'd suspect your timing chain is stretched.

Also, your vacuum readings are low at idle, they should be 18-22 if I recall. Could be a vacuum leak, which in turn is incorrectly advancing the engine (or not, as it seems).

Just my .02


Edit, here is a spec page from another sweptline site

http://www.sweptline.com/tech/engine2.html

[robertob]

Man that is really hard to follow. For one thing the cold behavior seems normal, or at least not unexpected. When the motor is cold and you are using the choke all kinds of things can happen.

In general though your idle timing should never be ATDC.

Here's what I would do first:

1. Check the timing marks on the pulley to make sure they are accurate with a screwdriver down the #1 plug hole. Sometimes the outer ring of the damper spins, or you could have a mixed up damper/front cover combination. Mark TDC on the pulley against the timing marks you use with whiteout.

2. Warm up the engine, let it get nice and warm so that it holds a steady idle. Set idle speed to 650 rpm plus or minus.

3. check timing and make sure it's reasonable.

4. Adjust idle mixture by the highest vacuum method. Re-adjust idle speed as needed to keep it at 650 rpm.

Beyond that I really don't know enough about holleys to give you specific advice but that should at least eliminate timing as an issue.

Your vacuum canister is connected to a ported vacuum source though isn't it?

[robertob]

Great advice also to check the timing chain. 10-15 degrees is usually the MAX I would ever allow on an engine of mine. 5 degrees is much preferred.

[wally426ci]

Ok, I don't have the mental capacity to concentrate on the above page, but let me say:

My timing would jump all over the place and you couldn't time it with a gun. I swapped out the timing set and it ran better, and you could time it!!

I yanked all the stuff off of the motor and painted it while i was at it and stuck on a new water pump too.

cheap fix!

[Dood]

By way of an update, I changed out the points and condensor last night for a Pertronix electronic ignition kit. Problem still persists. This wasn't really an attempt to solve it, just an upgrade I wanted. Gave me an opportunity to verify that the vacuum can was connected to a ported vacuum source.

Great stuff from you guys, thanks. Based on your comments (and a sleepless night), here is what I need to check:
1. Tachometer. Easy to check and a bad tach could be wreaking havoc on my idle and timing adjustments.
2. Search (again) for vacuum leaks. Double check brake booster. Never examined: on the driver's side of the intake manifold (from late 70s) located midway from front to back is a brass plate screwed into a box of some kind. I think this is some kind of EGR placeholder? I'm really clueless about this. Maybe it is a source of vacuum leak.
3. Accuracy of timing marks. Is balancer split? See above comments and those in Jumpy Timing Mark
4. Timing chain free play. Do at the same time as #3.